The term tian is often used for a French vegetable casserole, but a tian is actually a round baking dish filled with layered, overlapping vegetables and a fragrant sauce — similar to the better-known ratatouille. Whereas vegan ratatouille is made of chopped vegetables that are cooked on the stove, summer vegetable tian calls for very similar ingredients that are layered systematically in a baking dish and prepared in the oven.