Tonight, the blue-plate special. No, two blue-plate specials. A choice of meals designed for those rough times when you find yourself missing that one ingredient so important to all recipes-cold cash.

Well, in the first place, try not to let it get to you. There are some substitutes for that missing ingredient. Skill. Imagination. And maybe just a soupcon of courage.

In fact, there are some who would say that poverty was the best thing that ever happened to cooking. There's no trick to putting together a satisfactory little meal when you're starting out with a half-dozen lobsters or a rack of lamb. However, when you find yourself staring at a bag of beans, a box of rice, and last month's unpaid bills-that's when you've got to be a cook.

The careful reader will observe that there is not meat on tonight's menu. That is hardly accidental. Anyone forced to cook on a nonexistent budget soon learns the virtues of vegetarianism. However, don't apologize. Whichever main dish you choose to make tonight-and the chances are that no one in the family has ever tasted either one-is a guaranteed show-stopper. The rice-and-cheese casserole is a south-of-the-border sensation and the all-vegetable curry is the most popular dish in India; give either one a try and you'll understand why.

The Staples: For cooking both dinners, make sure these are all on hand: butter, milk, olive oil, wine vinegar, salt, pepper, cayenne pepper, cumin, dry mustard, tumeric, coriander, garlic.

The Shopping List: Again, for cooking both dinners: potatoes (3 pounds); frozen peas (1 box); brown rice; black-eyed peas; green chili peppers (small jar); ricotta cheese (1 small container); cheddar cheese (1 pound); fresh string beans (2 pounds); 2 large onions; lettuce; yogurt (1 small container); fresh fruit.

Blue Plate Special No. 1: Rice-and-Cheese Casserole.

5 P.M.: Cook 2 cups of dry brown rice, according to the directions on the package. This will take about 45 minutes. While the rice is cooking, cook 1 cup of black-eyed peas in water over medium-low heat until they are tender.

Remove both rice and black-eyed peas from heat when done. Then, in a large skillet over medium heat, melt a chunk of butter and then add a large onion that has been peeled and chopped fine. Add a cup of the green chili peppers, which have been cut into small pieces.

When the onions are cooked through, remove pan from the heat and set aside. Now mix in the rice and the black-eyed peas.

In a separate mixing bowl place the small container of ricotta cheese and add a splash of milk. Grate the cheddar cheese and add that. Stir together.

Butter the sides of a large casserole dish and add about a third of the rice mixture. An equal proportion of the cheese mixture. Another layer of rice. Another layer of cheese. And then bake the casserole for 40 minutes in a 350-degree oven.

Blue Plate Special No. 2: Potato-and-Pea Curry.

5:15 P.M.: Peel the potatoes and cut them into 1/2 inch cubes.

Using a large skillet, heat a 1/2 stick of butter over medium-low heat. As the butter melts, add the following spices: 1 teaspoon of cumin and another of salt, 1 teaspoon of tumeric and another of coriander and still another of dry mustard. Then add a 1/2 teaspoon of cayenne pepper, unless you like your food very hot; then you'll add a little more.

Using a wooden spoon, stir the spices together and allow them to cook gently for a few minutes. Then add the potatoes. Continue to stir while the cubed potatoes are being browned and seasoned on all sides. Cook them for about 10 minutes over medium heat.

When potatoes are browned, add 2 cups of water. The potatoes will simmer for about 20 minuees and then you'll add the frozen peas. In about 10 minutes or more, when the peas are cooked, add the container of yogurt and mix well. Dish will be ready in five minutes.

Other vegetables can be added at will; I find both chopped peppers and diced carrots welcome additions. The resulting curry is so pungent and flavorful that-despite its potato content-it can be served over rice.

Prepare Anytime: The green bean salad. Cut away the tips of the fresh string beans and cook the beans in boiling water until they are just tender. Drain the beans, cool them and slice them lenghtwise.

Slice a large onion into very thin slices and mix them with the cooked beans. Add salt, pepper, 1/2 cup of olive oil, the same amount of wine vinegar and a 1/4 cup of water. Plus 3 cloves of garlic, minced.

Mix this all together and store in refrigerator. Just before serving, put the beans on a few leaves of lettuce and serve.