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1
Arrange a rack in the middle of the oven and heat to 400.
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2
Fill the pasta pot with 6 quarts water, add 1 tablespoon salt, and heat to the boil.
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3
Shred the fontina through the larger holes of a hand grater, and toss the shreds with 1/2 cup of the grana (grated Grana Padano or Parmigiano-Reggiano).
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4
Put 3 tablespoons of the butter in the big skillet, and set it over medium-high heat.
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5
When the butter begins to bubble, drop in the mushroom slices, stir with the butter, season with 1 teaspoon salt, and spread the mushrooms out to cover the pan bottom.
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6
Let the mushrooms heat, without stirring, until they release their liquid and it comes to a boil.
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7
Cook the mushrooms, stirring occasionally, as they shrivel and the liquid rapidly evaporates.
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8
When the skillet bottom is completely dry, stir the half and half into the mushrooms, and continue stirring as the sauce comes to a boil.
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9
Cook it rapidly for a minute or two to thicken slightly, then keep it warm over very low heat.
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10
Meanwhile, stir the penne into the boiling pasta water and cook until barely al dente (still somewhat undercooked to the bite).
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11
Ladle a cup of the pasta cooking water into the mushroom sauce and stir.
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12
Drain the pasta briefly, and drop into the cream-and-mushroom sauce.
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13
Toss the penne until all are nicely coated, then sprinkle over them the remaining 1/2 cup of grana (not mixed with fontina) and the chopped parsley.
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14
Toss to blend.
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15
Coat the bottom and sides of the baking dish with the last tablespoon of butter.
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16
Empty the skillet into the dish, spreading the penne and sauce to fill the dish completely in a uniform layer.
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17
Smooth the top, and sprinkle the mixed fontina-grana evenly all over.
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18
Set the dish in the oven, and bake 20 to 25 minutes, until the cheese topping is crusty and deep golden brown and the sauce is bubbling up at the edges.
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19
Set the hot baking dish on a trivet at the table, and serve family-style.
