Preheat oven to 375°. Brush two large-rimmed baking sheets with 2 T olive oil each. Slice eggplant in half lengthwise then crosswise into 1/4”-thick slices. Slice the fennel and onion into 1/4”-thick slices. Arrange eggplant in single layer on one oiled sheet pan and fennel and onion on the other. Brush with olive oil. Sprinkle with minced garlic, 1/2 t salt and 1/4 t pepper. Roast in oven for 12-15 min, until vegetables are slightly softened but not browned.
Meanwhile, cut remaining vegetables into 1/4”-thick slices on the bias, crosswise. Brush the inside of a 2-quart oval baking dish with olive oil. Transfer the onion and fennel slices to the bottom of the dish.
Place vegetable slices on top of the onion and fennel, standing up, slightly overlapping and fitting tightly. Start on one side of the dish with rows of tomato, zucchini, eggplant, yellow squash and potato. Then repeat this order until all vegetables are used.
Drizzle with remaining olive oil, season with 3/4 t salt and 1/2 t pepper and sprinkle with chopped herbs. Garnish with thyme sprigs. Cover tightly with parchment and foil and bake for 75 min, making sure potatoes are done. Uncover and bake for 10-15 min longer, until lightly browned. Serve warm.
*Add another layer of flavor by topping the Tian with cheese. Sprinkle with 1/4 c of grated Gruyère or crumbled goat cheese for the last 10-15 min of baking.