Immediately after cooking in liquid, polenta is soft and creamy and can be served as is, usually as a bed for stews or sauces. Or it can be allowed to set up in a shallow pan or bowl and then cut into slices or wedges. These firm pieces can be grilled or fried and used in place of bread for crostini or layered, as in this dish, and baked with a meat sauce and cheese. Other cheeses, such as fontina or young pecorino, can replace the mozzarella, and a plain tomato sauce or mushroom sauce can stand in for the ragu. Although polenta is usually associated with northern Italy, this is a southern Italian dish. It's perfect to double or triple for a crowd, and can be assembled a day or two ahead of time and baked just before serving.