If desired, peel the eggplant. Cut into 1/2-inch slices; halve each slice. In a kettle cook the eggplant, zucchini, and salt in a small amount of boiling water for 4 minutes. Drain; pat dry with paper towels.
Divide the eggplant and zucchini among 4 individual au gratin dishes or casseroles. Top with ricotta cheese and tomato pasta sauce or spaghetti sauce. Top with the sliced tomato. Sprinkle with the mozzarella and Parmesan cheeses. Bake, uncovered, in a 350 degree F oven for 20 to 25 minutes or until heated through. Makes 4 servings.