• Make-Ahead

Ingredients

1. 1 stick (4 ounces) unsalted butter
2. 1/4 cup plus 2 tablespoons all-purpose flour
3. 1 quart whole milk
4. 1/2 cup homemade or store-bought tomato sauce
5. Pinch of freshly grated nutmeg
6. Salt and freshly ground pepper
7. 10 ounces white mushrooms, stems discarded and caps coarsely chopped
8. 1 medium onion, finely chopped
9. 1/4 pound thickly sliced prosciutto, finely chopped
10. 1 pound rigatoni
11. 1/2 pound imported Fontina cheese, coarsely shredded

Directions

1. Preheat the oven to 350°. In a large saucepan, cook 6 tablespoons of the butter over moderately high heat until lightly browned, about 2 minutes. Whisk in the flour and cook over high heat, whisking constantly, until foaming. Gradually whisk in the milk and bring the white sauce to a simmer over moderate heat, whisking frequently. Continue cooking until the sauce is thickened and no floury taste remains, about 10 minutes. Stir in the tomato sauce and nutmeg and season with salt and pepper. Remove from the heat.
2. Melt the remaining 2 tablespoons of butter in a large skillet. Add the mushrooms and cook over high heat, stirring occasionally, until dry and just beginning to brown, about 8 minutes. Add the onion and cook, stirring frequently, until softened, about 5 minutes longer. Stir the mushrooms and the prosciutto into the sauce. 
3. In large pot of boiling salted water, cook the rigatoni until barely al dente, about 8 minutes. Drain the pasta and return it to the pot. Add the tomato sauce and all but 1/2 cup of the Fontina and toss well. Transfer the pasta to a 9-by-13-inch glass or ceramic baking dish. Sprinkle the reserved 1/2 cup of Fontina over the top. 
4. Cover the pasta loosely with foil and bake for about 30 minutes, or until heated through. Remove the foil and bake until the pasta is golden, about 15 minutes longer.

Make Ahead

The recipe can be prepared ahead through Step 3. Cover the baking dish with plastic wrap and refrigerate the pasta overnight. Bring to room temperature before baking.