Saute the onion in the oil in a saucepan until the onion is tender. Add the tomato paste, water, oregano, parsley and salt. Simmer over low heat for 10 minutes. Cut the eggplant (or zucchini) into 1/4-inch-thick slices.
Arrange one layer of eggplant slices in the bottom of a lightly-oiled 9- by 13 inch baking pan. Pour on about 1/3 cup of the tomato sauce. Top with the Swiss cheese slices.
Add another layer of eggplant slices and pour on about 1/2 cup of the tomato sauce. Combine the rest of the sauce with the bread cubes and spoon over the eggplant. Sprinkle on the Parmesan Cheese. Bake in a 325-degree oven for about 25 minutes.