Heat 4 tbsp. oil in a large skillet over medium-high heat. Add onions. Season with salt and pepper. Cook until softened. Add thyme, pepper and garlic. Keep cooking for a minute or two more.
Pour the soft onions into your baking dish. Channel your inner artist! Create a small veggie stack by layering zucchini, yellow squash, eggplant and tomato slices. Set this stack on top of your onions, so they rest along the edge of the dish. Keep making stacks until the baking dish is filled and packed tightly together. Add salt, pepper and rest of the olive oil.
Bake uncovered for an hour, or until the vegetables are tender. For best results, cool to room temperature before serving. As you cool the tian, all of those wonderful, fresh seasonal flavors will come together.
Serve with a chiffonade of basil, fresh parsley or another garnish of your choice. Enjoy!