How to Make It
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.
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.
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.
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.