Step 1

An hour before you plan to cook, salt the eggplants to remove any trace of bitterness: using a mandolin slicer or very sharp knife, cut the eggplants crosswise into 1/8-inch (3 mm) rounds. Put them in a colander, sprinkle with 1 teaspoon salt, and toss to coat. Let rest for 1 hour to allow some of the moisture to be drawn out of the slices. Pat dry with a clean kitchen towel. Transfer to a bowl and sprinkle with 1 teaspoon of the herbes de Provence.

Step 2

Cut the zucchini and tomatoes crosswise into 1/8-inch (3 mm) rounds. Place in two separate bowls and sprinkle each with 1/2 teaspoon salt and 1 teaspoon of the herbes de Provence.

Step 3

Preheat the oven to 350° F (175° C).

Step 4

Lightly oil an 8×10-inch (20×25 cm) glass or ceramic baking dish. Scatter the sliced onions evenly over the bottom. Sprinkle with 1/4 teaspoon salt and a touch of olive oil.

Step 5

Arrange a row of overlapping tomato slices along one side of the dish. Pack them in tightly so that they are almost upright. Sprinkle with a little sage and garlic. Follow with a row of overlapping eggplant slices and then a row of zucchini slices, sprinkling each with a little sage and garlic as you go. Repeat the pattern until you’ve filled the dish and used up all the vegetables, packing the rows of vegetables together very tightly. If you have vegetables remaining at the end, slip them among their peers to flesh out the rows that seem to need it.

Step 6

Drizzle with 3 tablespoons olive oil, cover loosely with foil, and bake for 30 minutes.

Step 7

Increase the oven temperature to 425°F (220°C) and bake for another 30 minutes.

Step 8

Remove the foil and bake until the vegetables are tender and the tips of the vegetable slices are appealingly browned, about another 30 minutes. Serve hot, at room temperature, or chilled.