Preheat the oven to 160 °C / 320°F. Place in your oven a bowl of water to bring humidity into your oven (it's optional).
Heat 2 table spoons olive oil in a pan. Slice the onions. Fry them on a fairly high heat for about ten minutes. Do not forget to season with salt.
Meanwhile slice the vegetables (eggplant, zucchini, tomato). Cut them into slices of the same thickness.
Peel garlic clove and grate the bottom and edges of the baking dish with garlic. Chop the rest of this clove and put it in the frying onions.
Pour the onions in the bottom of the dish. Optionally pour some tomato sauce over.
Arrange vegetables slices on the edge, alternatively eggplant, zucchini and tomatoes. Pack well your rows and make sure the slices are the same size (possibly cutting the larger ones or adjusting 2 of the smaller ones).
Season with salt and pepper. Sprinkle with herbes de Provence and generously dizzle olive oil. You can optionally add a bit tomato sauce between the rows.
Cook at low heat, 160 °C/ 320°F, for 1:30. You can cover with aluminum foil the first hour. During the last half hour, occasionally sprinkle with cooking juices.