Guests on low-cholesterol diets present problems for the unprepared cook who, lacking the knowledge to accommodate the special diet, might prefer cooking for Scrooge.

But Bertha Rasa, a Bethesda resident, has lived with a low-cholesterol diet for 14 years. With a husband and six children who have followed the diet, too, Rasa can say from experience that it doesn't require a lot of special preparation.

It does require a little attention, though. She's careful to serve mostly fish, chicken and turkey at all family meals. The skin comes off her broiled chicken before it's eaten, and the fat is carefully trimmed from red meats when they are served.

Whip potatoes with evaporated skimmed milk (instead of cream) and a little margarine and "You couldn't taste anything better," says Rasa. She makes a lot of soups, but cools them and removes the hardened fat before serving, and she says she's "a big one for cooking fresh vegetables and eating fresh fruit."Like many hostesses, she'll browse through cookbooks to find recipes suitable for entertaining. Often she peruses the low-cholesterol books, "to see what I could serve that would be interesting and pretty. Food has to look good as well as taste good."

Changing a common diet to one that is low in cholesterol is a little more complicated than substituting margarine for butter on the broccoli or skim milk for cream in sauces. Most cheeses and egg yolks are morsels non gratis on the low-cholesterol diet. And there are hidden fats that cooks should look out for, says Nancy Ernst, nutrition coordinator of the National Heart, Lung and Blood Institute of the National Institutes of Health. First, non-dairy anything is usually harboring some kind of saturated fat inappropriate for a special diet. Non-dairy creamers, whipped topping, cream cheese and sour cream are usually laden with saturated coconut or palm oil. Likewise, labels of crackers and prepared baking mixes often reveal such ingredients as palm oil, coconut oil, hydrogentated, lard, cocoa butter and vegetable shortening (often solid palm or coconut oil).

Ernst advises replacing sour cream with yogurt. "You can make yogurt a vegetable-meat sauce or dessert sauce just by the flavoring you add," she says. Herbs, onions and chives make it a savory sauce. Honey, vanilla, fruit or spices make it a good topping for dessert. Yogurt, she points out, can also be used in cake recipes calling for sour cream.

The typical low-cholesterol diet requires a simultaneous reduction in all fat and calories. Rasa is sure to follow these guidelines when she eats out. She takes small portions of the food that could ruin her diet and tries to avoid eating dessert. "You have to be sensible," she says.

The following two recipes are from "Live High on Low Fat" (Lippincott; out of print), by Sylvia Rosenthal.

CREAM OF POTATO AND LEEK SOUP (6 servings) 4 leeks 3 tablespoons oil 1 medium-sized onion, thinly sliced 3 medium potatoes, peeled and sliced 3 cups chicken broth 1 teaspoon salt White pepper 2 tablespoons flour 1 quart skim milk Chopped chives

Trim the leeks of their roots. Trim the tops, leaving about 2 1/2 inches of green attached to the white portion of the leek. Split them to about 1/8 inch from the root end so that you can wash out any sand that has lodged there. Slice the leeks into fine slivers. In a heavy pan, heat the oil and cook the leeks and onion until they begin to turn golden. Add the potatoes, chicken broth, salt and pepper. Cover and cook slowly for 40 minutes.

Rub the mixture through a fine sieve or food mill. Return the pure'ed mixture to the pot. Make a smooth paste with the flour and 1/2 cup of the milk. Add the flour mixture and remainder of the milk to the pure'e, stirring constantly with a wire whisk until the soup is thickened and smooth. Taste to correct seasoning.

To be assured of a perfectly smooth, creamy soup, rub the mixture once again through a fine sieve. Serve sprinkled with chives.

STUFFED BREAST OF VEAL (8 to 10 servings) 4- to 5-pound veal breast, boned (reserve bones) 1 teaspoon salt Freshly ground pepper 3 tablespoons oil 1 onion, chopped 2 stalks celery, 1 with leaves, finely chopped; 1 whole 1 clove garlic, crushed 2 tablespoons minced parsley 2 cups soft bread cubes or crumbs 1 egg white, unbeaten 3/4 cup chicken broth 1 onion, thinly sliced 1 bay leaf 3 sprigs parsley 1 tablespoon oil 4 to 5 carrots, scraped and cut in halves 2 tablespoons tomato paste 1/2 cup dry white wine

Trim as much fat and sinew from veal as possible. Spread out flat, skin side down, and sprinkle lightly with 1/2 teaspoon of salt and a few grinds of pepper.

In a large, heavy skillet, heat the oil and cook the chopped onion, celery stalk with leaves, and garlic until onion is limp. Add the minced parsley and bread cubes or crumbs and cook for a minute. Remove from the heat and toss with the egg white, 3 tablespoons chicken broth and remaining salt and pepper. Mix well and spread this mixture over the veal. Roll the veal from the wide edge and tie securely with string.

Lightly oil a roasting pan and make a layer of the celery stalk, onion slices, bay leaf and parsley sprigs at the bottom. Place the veal, seam side down, on top of the vegetables. Lightly salt and pepper the roast if desired and dribble a tablespoon of oil over the meat. Arrange the bones and carrots around the roast.

Place in a 425-degree oven for 20 minutes, or until the top of the roast is just slightly brown. Combine tomato paste, remaining chicken broth and wine and pour over the veal. Reduce the heat to 350 degrees and cook for about 2 hours, or until the meat is tender. After the first half-hour of cooking, turn seam side up, bake 30 minutes, then turn seam side down for the balance of the cooking time. Baste frequently during the entire cooking time. Add more broth or wine if the sauce seems to be drying out. If the roast gets too brown, cover it with aluminum foil.

When the roast is done, remove the strings, slice in 1/4-inch slices and transfer to a warmed serving platter. Strain the sauce, pour it over the veal and garnish with carrots.

The following two recipes come from "Eat to Your Heart's Content" by Kay Beauchamp Heiss and C. Gordon Heiss (Chronicle Books; out of print).

RAINBOW DIP (Makes about 2 cups) 12 ounces large curd cottage cheese 2 tablespoons skim milk 1 teaspoon lemon juice 1/2 teaspoon salt 1 teaspoon horseradish 1 chopped carrot 3 radishes, halved 3 sprigs parsley

Rinse the cottage cheese in a sieve under running water and place in blender or food processor with milk, lemon juice, salt and horseradish; run on high until smooth. Add the carrots, radishes and parsley; blend until the vegetables are chopped.

CRANBERRY RELISH (Makes 4 cups) 1 lemon 1 pound fresh cranberries 1 to 1 1/4 cups sugar (or to taste) 1 cup crushed pineapple

Remove the end pieces of the lemon and discard. Grind the lemon with cranberries in food grinder or food processor. Add the sugar and pineapple. Chill overnight to allow flavor to develop.