Joan Mondale hates to cook. She makes no secret of that fact. But she loves people. So last week she had the best of both worlds. She and Bess Abell, who was her administrative assistant during the Carter administration, gave a lunch in honor of Eleanor Brenner. . . The guest of honor did all the cooking.

Eleanor Brenner has been on a low sodium diet for 15 years, long before it was "fashionable." She has gathered together her accumulated experience in a book of 300 recipes, "Gourmet Cooking Without Salt," (Doubleday, $15.95).

Most people know Brenner as a dress designer, specifically of Joan Mondale's inaugural outfit as well as other clothes she wore when her husband was vice president.

This other career as a cook and creator of low-sodium recipes comes as a surprise, such a surprise to some that they wondered who put Eleanor Brenner's cookbook together! Eleanor Brenner did, after many, many years of experimenting. She also uses low sodium varieties of many products such as cheese and canned tomatoes, but she makes her own yogurt, sour cream, myonnaise and sauces to keep them salt-free. She uses a salt substitute but its use in her recipes is optional.

No, it's not as easy as cooking with convenience foods, Brenner is the first to admit. But, she says, when you consider the alternative. . .

The bread served at lunch, light but dense, if that's possible, was made without salt, of course, but no one missed it. CRUSTY PEASANT RYE BREAD (Makes one loaf) No kneading 2 1/2 cups rye flour 3 cups unbleached flour 2 packages active dry yeast 3/4 teaspoon freshly ground black pepper 2 teaspoons seasoned salt substitute (optional) 1 teaspoon dry mustard (optional) 1/4 cup brown sugar, tightly packed 2 tablespoons honey 4 to 5 teaspoons caraway seeds 2 talespoons corn oil 1 cup milk (or low sodium dry milk) 1 cup water

Combine the rye and unbleached flours. In a large mixing bowl thoroughly blend 1 1/2 cups of the flour mixture with the yeast, pepper, salt substitute, mustard, brown sugar, honey and caraway seeds. In a heavy saucepan, over low heat, heat the oil, milk and water to warm (approximately 125 degrees) and stir well. Using an electric mixer on low-medium speed, slowly add the milk mixture to the flour-yeast misture, beating for 3 minutes, while scraping the sides of the bowl with a rubber spatula. Slowly add 3/4 cup of the remaining flour while beating at low-medium speed. Increase3 the speed to high and beat 3 minutes more. Stir in enough of the remaining flour mixture to make a stiff dough. Cover the dough with a turkish towel, put it in a wrm, draft-free place (approximately 85 degrees) and let the dough rise until double in bulk (approximately 45 minutes.)

Preheat oven to 375 degrees. Spoon the dough into a well-buttered and lightly floured lef pan (9-by-5-by-3-inches). Baker 40 to 45 minutes, or until the crust is well browned. Turn the bread out onto a metal rack and allow to cool at least 45 minutes before serving.