Tonight's menu serves three kinds of people: Those who are vegetarians, those who are thrifty and those who are hungry. If you happen to be all three, so much the better.

The meal revolves around 35 cents worth of lentils, one of the few bargains left in the marketbasket. In 45 minutes, the lentils become a hearty soup, and their incomplete protein is shored up by a cooling yogurt-based dessert and a crusty rye bread. The whole meal serves four amply for about $3.50. If you must spend your money, add a spiced cream cheese to spread on the bread for another $1.

With the exception of some straightforward vegetable chopping, the meal is simple enough to be prepared by a child, should you suddenly find one volunteering. Preparation takes about 15 minutes, and except for turning on the oven for the bread, the next 30 minutes are yours, free. MENU Hearty Lentil Soup Rye Bread Spiced Cream Cheese Banana Cream HEARTY LENTIL SOUP (4 servings) 1/2 pound lentils 2 1/2 cups water 1 carrot 1 stalk celery Onion 1 clove garlic 1/4 cup parsley 1 tablespoon oil 1 can (1 pound) tomatoes, undrained and chopped

Wash and pick over lentils. Cook in 2-quart saucepan with water to cover. Chop carrot and celery, add to lentils. Chop onion and garlic, cook in oil with parsley until onions are soft. Add onion/garlic mixture and tomatoes with liquid to lentils. Cook total of 45 minutes until lentils are soft. Add salt, if desired. BANANA CREAM (4 servings) 2 bananas 2 cups plains yogurt 1/4 cup mustard seeds

Slice bananas and add to yogurt with mustard seeds.

Grocery list: 1/2 pound lentils (or buy 1 pound package and use half), I pound can tomatoes, 1 pint container plain yogurt, 1 small jar mustard seed, 2 bananas, 1 small loaf uncut rye bread, 1 bunch parsley.

Staples on Hand: 1 onion, 1 carrot, I celery stalk, garlic, oil.

Game Plan: Wash and pick over lentils, put in pan with water and bring rapidly to a boil. Chop carrot and celery, add these to lentils. Peel and chop the onion and garlic, and saute these in a little oil with the parsley until the onions are soft.

Meanwhile, slice bananas into a bowl and add the mustard seeds and yogurt. Stir until mixed (children enjoy making this dish). m

When the onions are soft, add them to the lentil mixture. Open the can of tomatoes and use your knife to chop them in the can; add these, undrained, to the lentils. Cook the soup over medium heat for about 30 to 45 minutes.

About 10 minutes before serving, heat the bread in a 400-degree oven. Set out the cheese, if you have it, with some bread plates; pour the drinks, and ladle the soup.

My family likes to eat the yogurt dish alongside the lentils to cool our palates and inspire our children's appetities.