Begin by adding half of the mozzarella onto one slice of bread. Then add the cooked spaghetti, the marinara sauce, all of the meatballs, the pesto, and the rest of the mozzarella and Parmesan. Add the other slice of bread and set aside. In a small pan or on a Panini press, turn the heat to medium and melt 1 pat of butter until it begins to bubble. Once the bubbles begin to fade away, add the sandwich in and cook for a few minutes on each side until both are equally golden and brown. Once the cheese has melted and the crust has become completely crusty, rub the sandwich's exterior with the garlic clove like you would with bruschetta. Let the sandwich sit for a minute until the cheese has thickened up so it doesn't ooze out when you cut into it.