5 tablespoons unsalted butter
3 tablespoons all-purpose flour
2 1/2 cups half-and-half or whole milk
1 pound sharp Cheddar cheese, cut into 1/2-inch pieces
1/2 pound Colby cheese, cut into 1/2-inch pieces
1 tablespoon Dijon mustard
Pinch of freshly grated nutmeg
Pinch of cayenne pepper
Salt and freshly ground pepper
1 pound elbow macaroni
3/4 cup plain dry bread crumbs
Preheat the oven to 350°. Generously butter a shallow 2-quart baking dish. Melt 3 tablespoons of the butter in a large saucepan. Add the flour and cook over moderate heat for 2 minutes, stirring constantly. Add the half-and-half and cook over moderate heat, whisking constantly until thickened, about 3 minutes. Add one-half of the Cheddar and Colby cheeses and cook over low heat, stirring, until melted. Stir in the mustard, nutmeg and cayenne; season with salt and pepper.
Meanwhile, cook the elbow macaroni in a large pot of boiling salted water until al dente. Drain very well. Return the macaroni to the pot. Add the cheese sauce and the remaining cheese and stir until combined. Spread the macaroni in the prepared baking dish.