Macaroni Quattro Formaggi with Prosciutto

INGREDIENTS

• 6 tablespoons unsalted butter, plus more for dish
• 1 medium onion, finely chopped
• 1/2 cup all-purpose flour
• 5 1/2 cups whole milk
• 2 teaspoons coarse salt
• 1/4 teaspoon freshly ground black pepper
• 1 pinch of cayenne pepper
• 1 1/2 cups grated (about 4 1/2 ounces) Parmesan cheese
• 1 1/2 cups grated (about 6 ounces) Gruyere cheese
• 1 1/2 cups grated (about 6 ounces) Fontina cheese
• 1 1/2 cups (about 6 ounces) Gorgonzola cheese, broken into small pieces
• 1 pound ziti or mostaccioli pasta, uncooked
• 1/2 pound thinly sliced prosciutto