The one true test of any cook is a diet. This year's hot new diet is called the Scarsdale Diet, and it is the truest test of them all. I'm not talking about the breakfast, which consists of unbuttered toast, unsugared coffee and unsweetened grapefruit. I can think of only one sensible thing to do with a breakfast like that, and cooking isn't it.

The real challenge to the cook comes with the other two meals, the so-called lunch and the alleged dinner. One menu en the Scarsdale Diet, for example, recommends "cottage cheese, cabbage and two eggs." I can't get this combination out of my mind. What do you do with cottage cheese, cabbage and two eggs? A strange borscht? A sloppy salad? A disgusting omelet? What then? My feeling is that combinations like this one may well explain the diet's success. If that doesn't ruin your appetite, nothing ever will.

However, it shouldn't be necessary to go to those extremes. Here's meal that may stretch any diet a little bit, but it shouldn't totally destroy it.

The Staples: Make sure these are all on hand: salt, pepper, butter, basil, thyme, oregano, garlic, rosemary, savory.

The Shopping List: Two pounds of shrimp; whole wheat bread (one loaf); olive oil; 1 can beef bouillon; 1 bunch parsley; 5 large tomatoes; 1 bunch scallions; chives; carrots; celery; lemon; 3 grapefruits; grated parmesan cheese (4 ounces).

Prepare in Advance: The seasoned bread crumbs.

It is of course possible to buy readymade bread crumbs in the supermarket. But what you'll have here is high-nutrition bread crumbs.

The best starting point would be a loaf of whole wheat bread three or four days old. If the bread is still soft, dry it out in the oven. Spread the slices on a cookie sheet in a 250-degree oven and when the bread is thoroughly dry, put it into the blender along with the grated parmesan cheese and the following seasonings: a large pinch of chopped parsley, some chopped chives; a pinch of basil, another of rosemary and one of savory. Blend together and store in a cool spot.

Also Prepare in Advance: The shrimp. Shell the shrimps and remove the veins. The shells will come off easily and the veins can be removed with either a sharp paring knife or a toothpick. Then -- and here's an old dieter's trick designed to fool the eye -- slice the shrimps in half lengthwise.

Place the shrimp halves in a flat baking dish. Add 2/3 cup of olive oil (sorry about that). Chop up 3 cloves of garlic and 1/3 of the parsley and add that. Then, a large pinch of oregano. Some salt and pepper. And 1/4 cup of your seasoned bread crumbs.

Mix all this together and let it stand at room temperature for a couple of hours, or in the refrigerator overnight.

5:15 p.m.: In a pot with a tight cover melt 1/2 stick of butter (oops, sorry again) over medium heat. Add 1 cup of dry brown rice and stir it until it browns slightly in the butter. Now add 1/2 onion, chopped fine. And 2 1/2 cups of liquid (the canned bouillon and water). While you are doing this, eat 2 raw carrots. Rice pilaf is one of those experimental dishes that invites varied additions. Chopped mushrooms, diced peppers, chopped scallions, minced parsley, crushed tomatoes -- anything goes.

5:45 p.m.: Since we're using brown rice instead of white rice, the cooking time will be about twice the normal amount. Cover the pot tightly and cook the rice for about 45 minutes over a very low flame.

5:50 p.m.: And now, the tomatoes Provencal. Preheat oven to 350 degrees. Rinse and dry the 5 large tomatoes. Using a sharp knife, cut a circle out of the top of each tomato. Spoon the pulp of the tomato into a mixing bowl. And then turn the tomato shells over so that they'll drain while you're putting together the filling.

Chop up the rest of your parsley and add that to the tomato pulp. Along with 2 cloves of garlic, minced fine. A pinch of thyme. The juice of 1/2 lemon. Three scallions, chopped fine. A generous amount of salt and pepper. And enough of your seasoned bread crumbs to bind the mixture. Fill the tomato shells with the stuffing.

6 p.m.: Grease a casserole dish lightly, put the stuffed tomatoes in it and cover tightly. Place the covered dish in the preheated oven. Eat 2 stalks of celery.

6:30 p.m.: Increase oven heat to 450 degrees. Put the shrimps into the hot oven and remove the cover from the tomato casserole. Eat another carrot.

6:45 P.M.: Serve dinner. Go lightly with the shrimp but try not to cound calories. A few shrimps, a little rice, as much tomato as you want. And for dessert, a half grapefruit, no sugar. Why? That's just to keep you honest.