Serve this as a light lunch or summer supper, accompanied by a green salad, homemade bread, and white wine.
Sprinkle zucchini with (sea) salt and let stand for 10 minutes. Drain. Combine zucchini with lightly beaten egg yolks. Add mashed potato, sauteed onion, half the grated cheese, the garlic powder, and salt and pepper. Beat egg whites with pinch of salt until stiff. Fold into zucchini mixture. Pour into small casserole. Sprinkle remaining cheese on top. Bake at 350 degrees F for 30 to 40 minutes, or until brown. Serve hot.