1. Preheat oven to 375F. Butter 10 mini (1 cup) pie plates or a 9×13-inch baking dish. Melt 2 tbsp butter in a small saucepan over medium heat. Add breadcrumbs, and toast, stirring occasionally, until golden brown, about 5 minutes.
2. Warm milk in a medium saucepan over medium heat. Meanwhile melt remaining 6 tablespoons butter in a large high-sided skillet over medium heat. Cook until butter is bubbling. Add flour. Cook, stirring, for 1 minute.
3. Gradually pour warm milk into butter mixture, whisking constantly, until mixture bubbles and thickens, about 5 minutes. Remove from heat, and stir in 3 cups cheddar, 1 cup Gruyere, 1 cup fontina, 1/2 teaspoon salt, 1/4 teaspoon black pepper, the nutmeg, paprika, and cayenne pepper.
4. Bring a large pot of water to a boil. Cook pasta until al dente. Drain, and transfer past to cheese sauce. Stir to coat. Divide mixture among mini pie plates or to baking dish. Scatter remaining cup of cheddar, 1/2 cup Gruyere, and 1/2 cup fontina over top, and sprinkle with toasted breadcrumbs. Bake until cheese is melted and pasta is warmed through, about 15 minutes. Transfer to a wire rack and let cool for 5 minutes before serving.