Step 1Heat the oven to 425 degrees. Wash the zucchini, eggplant and tomatoes and wipe them dry. Cut them and the onions into slices about three-eighths-inch thick (if using a large eggplant rather than two smaller ones, cut it in half lengthwise before slicing). Keep all the vegetables separate from one another.
Step 2Butter a large baking dish (preferably earthenware or enameled cast iron). Cover the bottom of the dish with the onions. Then make a layer of zucchini and sprinkle with a little garlic, thyme, bay leaf, salt and pepper. Next, layer the eggplant, then the tomato. Season each layer as you did the zucchini.
Step 3Pour the oil over the surface of the vegetables and bake until golden brown, about 50 minutes to 1 hour. (If the vegetables dry out during the first 30 minutes of baking, cover them with foil; remove the foil during the last 10 minutes of baking.)