Preheat the oven to 400˚F (200˚C).
Place the onion slices and minced garlic in the bottom of a 5- x 7-inch (13 x 18 cm) baking dish. Sprinkle with 1 tablespoon of the olive oil, the 1/2 teaspoon flaky sea salt and some freshly ground black pepper.
Stack the eggplant slices upright against the long side of the dish so they are slightly overlapping each other. They should be quite tightly packed. Follow with a row of zucchini slices, arranged in the same manner. Next, make a row of tomato slices.
Continue in this manner until you have no more vegetable slices left. You should have enough vegetable slices and room to make at least two rows of each vegetable.
Drizzle 1 tablespoon of olive oil over the vegetables, sprinkle with the Herbes de Provence, cover the dish with aluminum foil and bake for 45 minutes.
Remove the foil from the dish, drizzle with a little more olive oil and bake, uncovered, for a further 20 to 30 minutes, until the vegetables are cooked through.
Season to taste. Serve warm or at room temperature.