1) Preheat the oven to 190°C/gas mark 5.
2) Heat the butter and 30ml of olive oil in a large saute pan, add the mushrooms, and cook over medium heat for 3 to 5 minutes, until they are tender. Add the sherry and continue to saute for a few more minutes, until the sherry is absorbed. Set aside.
3) Bring a large pot of water to a boil and add a splash of olive oil and a pinch of salt. Add the pasta and cook for 6 to 8 minutes, until al dente. Drain well.
4) Meanwhile, melt the truffle butter in a large saucepan and whisk in the flour. Cook for 2 minutes over low heat, stirring constantly with a whisk. Slowly whisk in the hot milk and cook for 2 minutes, stirring constantly with a wooden spoon, until the white sauce is thickened and creamy. Off the heat, add the Gruyere, Cheddar, 1 1/2 tbsp salt, the pepper, and nutmeg.
5) Combine the pasta, sauce, and mushrooms in a large bowl and pour them into a 25x35x5cm baking dish.
6) Place the garlic and parsley in the bowl of a food processor fitted with the steel blade and pulse until they're minced. Add the bread crumbs and pulse to combine. Sprinkle the crumbs over the pasta and bake for 35 to 45 minutes, until the sauce is bubbly and the crumbs are golden brown. Serve hot.