"Let's make a division between American Mexican and true Mexican. If we keep them apart then we won't quarrel," suggests an authoring on Mexican cooking, a woman who isn't even Mexican.

English-born Diana Kennedy begins most of her cooking demonstrations with that caveat. It prevents arguments, especially when she says things like: "American-Mexican is just plain hot, period. It has one texture, too much sour cream, too much cheese and too much cumin in the chorizo."

She elaborated further on the differences, trying to hide her disdain for "North-of-the-border Mexican": "Mexcians don't use black olives. Ground meat is rarely used; it's shredded instead. The average menu in the United States is a combination plat with an all-purpose sauce. I prefer to forget the Texas table sauce which destroys your palate for anything else. The Mexicans would never mix a taco with an enchilada and they make a different sauce for each thing."

Kennedy thinks Americans have the skewed impression that all Mexican food is hot because the part of Mexico that borders the United States is known for its red hot seasonings.

The lusher parts of Mexico, where food grows more abundantly have much milder, often more complex cooking that dates back a long way. "Mexican cooking has a pre-Columbian heritage. There was a tremendous cuisine when Cortez arrived. A great tradition; great regional food with many wild foods. People say, 'You do so exaggerate about Mexican food.'"

Kennedy disagress. But she admits the cooking is messy and not necessarily pretty. "Mexican food, " she told an audience of a newspaper food editors at their annual meeting in Chicago recently, "is mostly pot to mouth food. I always think of my kitchen as a taco stand.

"The most sophisticated food," Kennedy said, "is found in Mexico City. There are Indian and Colonial influences plus Miximillian, Italian and Hungarian."

On the other hand, she suggested, maybe the most sophisticated cooking is in the Yucatan. "I've seen many traces of Moorish - the escabeches (pickled dishes) and the way they mix their herbs and are using spices. It has the most stupendous variety of mushrooms."

How can an English woman be so positive about a cooking that is not her own? Because she "literally spent 20 years wandering (through Mexico), eating, dining in cheap hotels and getting fleabitten and because of the generosity of cooks in Mexico. I went to the markets with maids, into their kitchens. I measured things. I badgered grandmothers."

Her first book, "The Cuisines of Mexico," published in 1972 by Harper & Row, is said to be considered a classic by many Mexicans. It is also a primer for the enthusiastic, eager student of the cuisine. As Kennedy says "there's nothing difficult about the technique, but if you get into international cuisines you must be prepared to do a bit of homework. The most important step is learning the ingredients."

What makes Mexican cooking so exciting to Kennedy is what makes learning about the ingredients difficult: 200 different kinds of chiles, each with its own special flavor and hotness; 50 different kinds of bananas. So she recommends reading the introduction of her book. It's the next best thing to attending her cooking classes in Mexico, where she now lives full time.

She met her late husband, New York Times correspondent Paul Kennedy, while traveling around the Caribbean and moved to Mexicao when they married. Shortly before he died in 1967, they moved to New York, where she began teaching classes. But she made frequent trips back to Mexico to continue research on what has turned out to be her life's work: codifying, if such a thing is possible, the cuisines of Mexico.

Her second publication, "The Tortilla Book," a paperback also published by Harper & Row, deals exclusively with the tortilla in 80 incarnations. "Cuisines of Mexico, Vol. II" is scheduled for publication next year.

While she continues her research, Kennedy is also building "a small ecological house of adobe because it is one of the best insulaters." Two and a half hours west of Mexico City, it will have solar energy, a beehive oven, a barbecue, fireplace but no central heating and a refrigerator run by wind power.

Why? Because, Kennedy says, "that's the way we are going. We can live withoug exhausting the earth's resources. It will be a house of tradition and of the future."

Kennedy also wants it to be "a center of gastronomic study because a great number of indigenous recipes are disappearing too fast. I want to preserve them on the record, even if they are not readily translatable."

Something of a purist, Kennedy makes ingredient substitutions in the United States only under duress. Many times she simply says there are no substitutions.Forget about making the dish.

In recent years it has become somewhat easier to find ingredients, at least in large cities, but Kennedy says she still has not found an authentic Mexican restaurant anywhere in this country. It's difficult to quarrel with that assessment if you accept her original premise: American-Mexican bears little relationship to true Mexican. Which means if you want to savor real Mexican food you'll have to cook it yourself.

The following two recipes were demonstrated by Kennedy during the Food Editors' Conference. CHILE CON QUESO (6 servings) 20 fresh Anaheim chiles or 20 canned whole green chiles (4 or 5 four-ounce cans) 5 tablespoons peanut or saf-flower oil 1 medium onion, thinly sliced 1 medium tomato, skinned and thinly sliced 3/4 cup milk 3 tablespoons water 1/3 pound Meunster, Jack or mild Cheddar cheese, thinly sliced 1 1/2 teaspoons salt

Cut chiles into strips. Heat the oil in frying pan or flame-proof serving dish and cook the onion, without browning, until soft. Add tomatoes and chile strips. Cover and cook over medium heat about 8 minutes for fresh chiles, 4 minutes for canned. Add milk and water. Let mixture cook a few minutes more. Just before serving add cheese and salt and cook until cheese melts. Serve with hot tortillas.

TO USE FRESH CHILES: Chiles should be smooth, not wrinkled with age, firm, thick fleshed and, therefore, heavy. Place the chiles directly onto the flame of an open burner and let the skin blister and char slightly. Turn them from time to time - so that the flesh does not become burned through - until they are evenly charred. Place them immediately into a damp cloth or polyethylene bag and let them "sweat" for about 20 minutes. While the skin will be ready to come off long before that, this steaming process will also help to cook the chile a little more - but not overcook it. You could also roast the peppers under a broiler, turning as above. The lovely burned flavor enchances the taste of the chiles.

The skin will peel off easily. Slit each chile down the side, leaving the top ridge intact. Carefully cut out the core under the base of the stem; wash out the seeds and remove as much as you can of the veins without tearing the flesh of the chile. Dry the chile well with paper towering before using. CREPAS DE CAMARON CON SALSA DE CHILE PASILLA (Shrimp Crepes with Chile Pasilla Sauce) (6 servings) 1 1/2 pounds small, cooked shrimp 12 thin crepes, about 5 1/2 inches in a diameter, prepared according to any standard crepe recipe 6 fresh or dried chile pasilla 1 1/2 pounds tomatoes, broiled 1/4 medium onion 1/3 cup peanut or safflower oil 1/2 teaspoon sugar 1 1/2 teaspoons salt 1 1/2 cups thick sour cream 1 1/4 cups sharp Cheddar cheese, grated Sour cream for granish

Heat a griddle or comal and char the fresh chiles lightly, turning them from time to time so that they do not burn. Set aside to cool a little. When they are cool enough to handle, remove the seeds and veins being careful to remove them all or the sauce will be too sharp. If using dried chiles, refresh them on a hot griddle to soften. Without soaking the chiles, put them into a blender and blend together with the tomatoes, onion, sugar and salt to a smooth paste. If mixture sticks, add a little water.

Heat the oil in a frying pan but do not let it smoke. Add the sauce and cook the mixture over medium flame, stirring it from time to time so that it does not stick to the bottom of the pan. You will probably have to cover the pan with a lide, as the sauce splatters rather fiercely. After about 15 minutes the sauce will have thickened. Set it aside to cool a little.

Stir sour cream into the sauce and let it continue to heat through for a minute or so. Do not let the sauce boil or it will curdle.

Mix the shrimps into 1 cup of the sauce. Place a little of the mixture in each of the crepes and roll them up loosely. Place the crepes side by side in an oven-proof serving dish. Pour the remaining sauce over them.

Sprinkle the grated cheese over the sauce and put some dollops of sour cream around the edge of the dish.

Let the crepes heat through in the oven at 350 degrees until the cheese melts. Serve immediately.

The following two recipes are from "Cuisines of Mexico." PECHUGAS DE POLLO CON RAJAS (Chicken breasts with chiles) (6 servings) 6 small chicken breasts Salt and freshly ground pepper 1/4 cup butter 1/4 cup peanut or safflower oil 1 large onion, thinly sliced 2 1/4 pounds chiles poblanos* (or about 20 to 22 canned, peeled, green chiles) 1 cup milk 2 cups thick sour cream (see recipe) 1/4 pound cheddar cheese, grated

Remove bones and skin from the breasts and cut each of them into 4 filets. Season them well with salt and pepper. Heat the butter and the oil together and saute the chicken filets for a few moments on both sides until they are lightly browned. Set them aside. In the same fat, fry the oinion gently, without browning, unitl soft. Peel and clean the chiles. (See directions above). Set aside 3 of the chiles. Cut the rest into strips. Add these to the onions in the pan, cover and cook over medium flame (8 minutes for fresh chiles and 5 for canned chiles).

Blend the reserved chiles until smooth with the milk and 1/2 teaspoon salt. (If using canned chiles, use only 2/3 cup milk.) Add the sour cream and blend for a few seconds longer.

Arrange half of the chicken filets in an oven proof baking dish about 3 inches deep and about 10 inches in diameter. Cover them with half the strips and half the sauce. Repeat the layers. Sprinkle the cheese over the top and bake until the chicken filets are done and the cheese melted - it is not necessary to brown it - about 30 minutes.

Kennedy says the breasts can be sauteed, the strips cooked and the sauce blended ahead of time. The dish should be assembled just before baking.

*Fresno chiles, available in Washington markets, can be substituted for the pobalno chiles.

While commerical sour cream can be used, Kennedy prefers either making her own or using the French creme fraiche . THICK SOUR CREAM (About 1 cup) 1/2 pint heavy cream 2 tablespoons buttermilk

Put the cream and buttermilk into a jar or glass bowl and mix them well together. Cover with plastic wrap and set the mixture aisde in a warm place, but not too warm (a strong pilot lights is too warm; the cream will taste "cooked" and a skin will form) until it is set, about 6 hours. Refrigerate the cream overnight; it will thicken and become more solid. PESCADO EN CILANTRO (Fish in coriander) (6 first course or 4 main course servings) 3 to 3 1/2 pound red snapper* 1 scant teaspoon salt Freshly ground pepper 1 medium onion (about 6 ounces), finely slices 1/3 cup lime juice 6 tablespoons olive oil 3 chiles jalapenos en escabeche 3 tablespoons juice from the chile can 2 cups fresh coriander leaves, roughly chopped

Note: coriander is also known as Chinese parsley and is available in Oriental and Latin markets.

Have the fish cleaned, leaving the head and tail on. Prick the fish well on both sides with a coarse-tined fork, and rub it with the salt and pepper. Place the fish onto an ovenproof dish just large enough to hold it, with half the onions underneath and the rest on top. Pour the lime juice over it and set it aside for about 2 hours, turning it over once during that time.

Cover the dish with foil and bake the fish at 350 degrees for about 15 minutes on each side. Add the rest of the ingredients and continue cooking the fish, covered, until it is just cooked, basting it from time to time with the juices of the dish - about 20 minutes.

*If red snapper is not available, rock-fish can be substituted.