Preheat the oven to 350^° F.

Butter a 9- by 13-inch casserole dish with 1 tablespoon of the butter and set aside.    

Bring a large pot of salted water to a boil. Add the macaroni and cook until al dente, about 10 minutes. Transfer pasta to a colander and rinse under cold water until cool; set aside to drain and then add the noodles to a large mixing bowl.

In a large skillet over medium-high heat, cook the sausage, stirring, until browned and the fat is rendered. Remove with a slotted spoon and set aside to drain on paper towels. Pour off all but 1 tablespoon of fat from the pan. Add the onion and bell pepper, and cook, stirring as needed over medium-high heat until soft, about 3 minutes. Add the garlic and cook, stirring, for 1 minute. Remove from the heat.

Melt the remaining 8 tablespoons butter in a large, heavy saucepan over medium heat. Add the flour, and stirring constantly with a wooden spoon, cook over medium heat until toasty and fragrant, 3 to 4 minutes, being careful not to let the flour brown. Using a whisk, add the milk in a steady stream and cook, whisking constantly, until sauce is smooth and thickened, 4 to 5 minutes; remove from the heat. Add the salt, pepper, cayenne, and 2 cups of the cheese; stir well to combine.

Combine the cheese sauce with the macaroni and the reserved sausage mixture, and stir well to combine. Pour into the prepared baking dish.

In a mixing bowl, combine the remaining 1 cup of cheese with the breadcrumbs and sprinkle over the macaroni.  Bake until golden brown and bubbly, about 25 minutes.

Remove from the oven and set aside at least 5 minutes before serving.