Preheat the oven to 375 degrees.
Heat the butter and 2 tablespoons oil in a large (12-inch) sauté pan, add the mushrooms, and cook over medium heat for 3 to 5 minutes, until tender. Add the sherry and sauté for 2 to 3 minutes, until the sherry is absorbed. Set aside.
Meanwhile, add a tablespoon of salt to a large pot of water and bring to a boil. Add the pasta, cook for 6 to 8 minutes, until al dente, and drain.
Scald the milk in a medium saucepan. Melt the truffle butter in a large (4-quart) saucepan and whisk in the flour. Cook over low heat for 2 minutes, whisking constantly. Slowly add the scalded milk and cook for 2 minutes, stirring with a wooden spoon, until thick and creamy. Off the heat, add the Gruyère, Cheddar, 2 tablespoons salt, 2 teaspoons pepper, and the nutmeg.
In a large bowl, combine the pasta, the cream sauce, and two-thirds of the mushrooms and pour into a 10 × 13 × 2-inch ovenproof baking dish. Place the bread, garlic, and parsley in the bowl of a food processor fitted with the steel blade and process to make fine crumbs. Sprinkle the herbed crumbs evenly on the pasta and arrange the remaining mushrooms down the middle.
Bake for 35 to 45 minutes, until the sauce is bubbly and the crumbs are golden brown. Let stand for 5 minutes and serve hot.
NOTE: You can find white truffle butter at urbani.com; it keeps in the freezer for up to 6 months. Don’t substitute black truffle butter; the flavor is very different.
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