A tian is a French dish whose name-like that of the casserole and the terrine-refers to the food itself as well as the vessel in which it is cooked.
Preheat the oven to 400 degrees with a rack in the center. Coat the bottom of a 9-by-13-inch baking dish with 1 tablespoon oil.
Arrange the vegetables in the baking dish in slightly overlapping rows, alternating potato, eggplant, zucchini, and tomato. Season with salt and pepper and top with rosemary sprigs. Place garlic in center of dish. Drizzle all over with remaining 3 to 4 tablespoons oil.
Bake, swirling baking dish every 10 minutes to distribute oil, until vegetables are tender and garlic is soft, 40 to 50 minutes. This will keep the vegetable moist and tender.
Remove baking dish from oven. Remove garlic and let stand until cool enough to handle. Squeeze garlic pulp into a small bowl and mash with a fork. Spread over vegetables, or serve on toast, if desired.