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1
Preheat the oven to 350.
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2
In a large saucepan, cook 6 tablespoons of the butter over moderately high heat until lightly browned, about 2 minutes.
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3
Whisk in the flour and cook over high heat, whisking constantly, until foaming.
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4
Gradually whisk in the milk and bring the white sauce to a simmer over moderate heat, whisking frequently.
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5
Continue cooking until the sauce is thickened and no floury taste remains, about 10 minutes.
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6
Stir in the tomato sauce and nutmeg and season with salt and pepper.
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7
Remove from the heat.
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8
Melt the remaining 2 tablespoons of butter in a large skillet.
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9
Add the mushrooms and cook over high heat, stirring occasionally, until dry and just beginning to brown, about 8 minutes.
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10
Add the onion and cook, stirring frequently, until softened, about 5 minutes longer.
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11
Stir the mushrooms and the prosciutto into the sauce.
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12
In large pot of boiling salted water, cook the rigatoni until barely al dente, about 8 minutes.
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13
Drain the pasta and return it to the pot.
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14
Add the tomato sauce and all but 1/2 cup of the Fontina and toss well.
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15
Transfer the pasta to a 9-by-13-inch glass or ceramic baking dish.
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16
Sprinkle the reserved 1/2 cup of Fontina over the top.
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17
Cover the pasta loosely with foil and bake for about 30 minutes, or until heated through.
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18
Remove the foil and bake until the pasta is golden, about 15 minutes longer.
