Wrap a 1-inch cube of Fontina, mozzarella or provolone cheese or a halved hard-boiled egg with half a slice of prosciutto each. Form large 4-inch meatballs around each piece of cheese or egg.
Arrange balls on cooling rack set over a baking sheet and drizzle with oil. Roast until brown and crispy all over in a 400°F oven, 25-30 minutes. Serve 2 balls per person in shallow bowls sitting in a pool of tomato sauce, passing crusty bread at the table for mopping.
Cheese and/or egg may also be wrapped in spinach leaves or a combination of spinach and ham. If you’d rather not bother with stuffing the meatballs, roast large meatballs and simply serve with dollops of fresh ricotta cheese alongside or atop the meatballs.