Fusilli can be substituted for the rotini. Some popular brands of sausage come in packs that range from 14 to 19 ounces; this recipe will work well with any amount of sausage in that range. Use a 10-ounce block of American cheese from the deli counter rather than presliced cheese.

Adjust oven rack to upper-middle position and heat oven to 375 degrees. Bring 4 quarts water to boil in Dutch oven. Add pasta and 1 tablespoon salt and cook until just al dente. Drain pasta; set aside.

Add butter to now-empty pot and melt over medium-high heat. Add sausage and cook, breaking up meat with wooden spoon, until sausage begins to render its fat, about 2 minutes. Stir in bell peppers, onion, and salt and cook until sausage is no longer pink and vegetables are softened, 5 to 7 minutes. Transfer 1 cup sausage mixture to small bowl; set aside.

Stir flour into remaining sausage mixture in pot and cook for 1 minute. Stir in cream and milk, bring to boil, and cook until sauce is thickened, about 2 minutes. Add American cheese, 1 cup fontina, Parmesan, mustard, and pepper and cook, stirring constantly, until cheese is completely melted, about 1 minute. Off heat, stir pasta into sauce until thoroughly coated.

Transfer pasta mixture to 13 by 9-inch baking dish and spread into even layer. Sprinkle with remaining 1½ cups fontina; reserved sausage mixture; paprika; and fennel seeds, if using.

Bake until topping is lightly browned and filling is bubbling, 18 to 20 minutes. Let cool for 20 minutes. Serve.

TO MAKE AHEAD: Fully assembled casserole, prepared through step 4, can be wrapped tightly in plastic wrap and refrigerated for up to 2 days. When ready to serve, remove plastic and bake until heated through, 40 to 45 minutes.