It's been 10 years since Francis Moore-Lappe' wrote "Diet for a Small Planet," the vegetarian treatise on feeding the world that told readers how to combine foods to get maximum protein from minimum energy consumption.

In the past 10 years Americans have learned, however, that protein is the least of their dietary worries. New substances -- fiber, sodium and cholesterol, for instance -- have received a great deal of attention in the last few years.

But combining vegetable protein has an economic advantage, a feature with ever-increasing importance now that unemployment is soaring to new heights. And vegetable protein combinations are satisfying -- they're nourishing, comforting and filling and give the diner a sort of self-righteous feeling of conservation. If you must forego filet, it's nice to be rewarded with emotional gratification.

Vegetable protein combines in such interesting ways -- Mexican tortillas and beans, Mediterranean rice and nuts or sesame seeds and garbanzo beans, Chinese soybeans and rice, and American peanut butter and bread. It may not be steak but it sure beats hamburger.

Since lentils cook faster than other legumes and don't require soaking, they are the natural choice for a quick dinner. Lentils available in a supermarket have an unfortunate olive drab color, however, so serve them with something brilliant red, orange or yellow. The lentils below top any of your favorite grains. We put them over bulgur (whole wheat kernels) but they would go equally well with kasha (simmered buckwheat kernels), rice or millet.

Sometimes it takes special trips to the health food store to prepare healthful vegetarian meals. This one, however, only requires a trip through the express lane of the average supermarket.

Express lane list: lentils, onion, cider vinegar, honey, basil, bulgur or rice, yellow squash, tomato. SWEET AND SOUR LENTILS (2 to 4 servings) 2 1/2 cups water 1 cup lentils 1 small onion, chopped 3 to 4 tablespoons cider vinegar 2 tablespoons honey 1/2 teaspoon salt 1 teaspoon basil

Combine water and lentils and bring to a boil. Cover, reduce heat and simmer 1/2 hour. Add chopped onion and cook 15 minutes more. Add remaining ingredients, more water if lentils look dry, and continue to simmer for 10 minutes. If the lentils are not tender, add a little more water and cook a few more minutes. Serve over rice, bulgur, kasha or millet. SQUASH AND TOMATOES FOR FOUR

Peel 2 tomatoes, remove their cores and chop them. Combine them in small saucepan with 2 sliced summer squashes. Salt and add generous amount of freshly ground pepper. If you have oregano, add 1/4 teaspoon of that (it's optional). Cover the pan closely and steam over very low heat for 10 to 15 minutes or until squash is barely cooked. Serve hot.