Iconic Provençal summer gratin: alternating tightly shingled slices of zucchini, yellow squash, ripe plum tomatoes, and eggplant baked over sweet caramelized onions with olive oil, garlic, and thyme.
Sauté sliced onion and garlic in 2 tbsp olive oil until soft and golden. Spread across bottom of an oval gratin dish.
Arrange sliced zucchini, yellow squash, tomatoes, and eggplant in tight, upright alternating rows packed closely across the dish.
Drizzle generously with olive oil, scatter fresh thyme leaves, and season with fleur de sel and black pepper.
Cover with foil and bake at 180°C for 40 minutes, then uncover and bake for 25 minutes more until vegetables are caramelized at the edges and meltingly tender.