Preheat oven to 375° F. Butter eight 8-ounce ramekins and set aside.

Fill a 4 to 5 quart pot about three-quarters full with water and add 1 tablespoon of the salt. Bring to a boil and add the pasta. Cook, stirring once or twice, until tender but firm, about 4 minutes, and drain. Reserve the pot.

Put bread slices in a food processor and process until the crumbs are coarse. Set aside.In a medium nonstick skillet, heat 2 tablespoons of the butter over medium-high heat. Add bread crumbs and cook, stirring frequently, until toasted, about 3 to 4 minutes. Transfer to a plate. Wipe out the skillet.

Using the same skillet, heat 1 tablespoon of the butter over medium-high heat. Add chopped onion and cook, stirring occasionally, until soft and translucent, about 5 minutes. Add sausage and cook, stirring occasionally, until lightly browned and heated through, 8 to 10 minutes. Add black pepper to taste. Transfer to a paper towel-lined plate to drain. Set aside.

Using the same pot you used to cook the pasta, melt the remaining 2 tablespoons of butter over medium heat. Slowly whisk in the flour and stir constantly until a paste forms, 3o to 45 seconds. Continue stirring for 1 to 2 minutes more, until the mixture starts to darken slightly and smell a bit nutty. Slowly whisk in the milk, cream, and the remaining 1 teaspoon of salt. Cook until the mixture starts to thicken and is just beginning to bubble around the edges, 7 to 9 minutes. It should be thick enough to coat the back of a wooden spoon. Add 2 cups of the Colby and all of the Fontina, and cook until the cheeses have melted and the sauce is smooth but not too runny. If it's soupy, just continue cooking until it thickens. Then add the pasta, onion, brats, mustard, and cayenne. Stir just until incorporated.

Divide evenly between the prepared ramekins. Sprinkle with the remaining 1 cup of Colby and top with the bread crumbs.

Place the ramekins on a rimmed baking pan and bake for about 25 minutes, or until the mixture is brown and bubbly. Let cool for 10 to 15 minutes before serving.