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1
In a large, deep skillet, melt 4 tablespoons of the butter.
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2
Add the onion and cook over low heat, stirring occasionally, until softened but not browned, about 10 minutes.
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3
Using a slotted spoon, transfer the onion to a bowl; leave as much of the butter in the pan as possible.
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4
Add the mushrooms to the skillet, season with salt and pepper and cook over high heat, stirring occasionally, until the liquid is evaporated and the mushrooms are golden, about 8 minutes.
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5
Stir in the shallot and garlic and cook for 1 minute.
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6
Return the onion to the skillet.
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7
Add the chicken stock and parsley and simmer over moderately high heat until the liquid is reduced to 1/4 cup, about 8 minutes.
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8
Add the remaining 2 tablespoons of butter along with the pistachios, lemon juice and chile and stir until the butter is melted.
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9
Season with salt and pepper.
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10
Meanwhile, cook the pappardelle in a large pot of boiling salted water until al dente.
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11
Drain the pasta, reserving 1/2 cup of the cooking water.
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12
Add the pappardelle to the skillet along with the reserved cooking water and the olive oil.
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13
Cook over moderately high heat, tossing, until the pasta is coated with a thick sauce, about 2 minutes.
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14
Transfer the pasta to bowls and serve, passing the cheese at the table.
