Chop enough of the eggplant to make 1 cup. Halve remaining eggplant lengthwise and cut into thin slices; set aside.
In a medium skillet cook onion and garlic in 1 tablespoon of the oil until tender but not brown. Add chopped eggplant, chopped tomatoes, oregano or thyme, sugar, salt, and pepper. Cook, uncovered, over medium-low heat about 15 minutes or until liquid has evaporated and mixture is of spreading consistency, stirring occasionally.
Place the bread shell on a lightly greased or parchment-lined baking sheet. Spread the warm tomato mixture over bread shell. Arrange the tomato slices and eggplant slices on the tomato mixture. Arrange zucchini slices and summer squash slices on top. Brush sliced vegetables with the remaining oil. Sprinkle with feta cheese and olives. Sprinkle with mozzarella cheese.
Bake in a 400 degree F oven for 12 to 15 minutes or until cheese melts and vegetables are heated through. Makes 12 appetizer servings.