Cut the zucchini lengthwise into slices about ¼ in. (6 mm) thick.
If the eggplant is large, with a thick skin, peel it. Cut large eggplant crosswise into slices ¼ in. (6 mm) thick. Cut small eggplants lengthwise to make larger slices.
Core and seed the peppers. Cut into quarters lengthwise.
Cut the onion crosswise into slices about 1/3 in. (