Peel and cut the onion into half moons. Peel and cut the garlic into 2-4 pieces.
In a medium skillet heat 2-3 Tbsp of olive oil until hot, add onion and sauté until tender, stirring frequently. Add garlic and cook for another 2 minutes. Season with salt and pepper. Remove from the heat.
Preheat the oven to 400℉. Spread the cooked onion on the bottom of a baking dish.
Cut zucchini and summer squash into 1/4 inch slices.
Cut the eggplant into 1/4 inch slices.
Core and cut tomatoes into 1/4 inch thick slices.
Arrange the zucchini, eggplant, squash and tomatoes in alternating rows. Continue until the baking dish is filled.
Sprinkle the vegetables with salt, pepper, Herbs de Provence and drizzle with the remaining olive oil.
Bake for 40-50 minutes or until the vegetables are soft and golden. If the vegetables are not ready, but the top is starting to burn, cover the tian with aluminum foil.