Sauté the mushrooms: Heat the olive oil and butter in a large sauté pan over medium heat. Add the mushrooms and cook until they've released their liquid and are nicely browned. Stir in the cream sherry and continue cooking until most of the sherry has evaporated, about 1 minute. Transfer the mushrooms to a bowl, reserving 15–25 slices for garnish if desired.
2 Tbsp olive oil, 2 Tbsp butter, 1 pound cremini mushrooms, 3 Tbsp cream sherry
Cook the pasta: Bring a large pot of generously salted water to a boil. Cook the pasta until just al dente (about 6–7 minutes for cavatappi). Drain well and set aside.
1 pound pasta
Make the cheese sauce: Using the same sauté pan, melt the truffle butter over low heat. Whisk in the flour and cook for 1 minute. Slowly whisk in the warm milk until smooth. Continue cooking, stirring constantly, until the sauce thickens and becomes creamy.
3 ounces white truffle butter, ½ cup all-purpose flour, 1 quart whole milk
Add the cheese: Reserve ½ cup of the shredded Gruyère for the topping. Remove the pan from the heat and stir in the remaining Gruyère, all of the cheddar, salt, pepper, and nutmeg until the cheese is completely melted and the sauce is smooth. Preheat the oven to 375°F.
1½ Tbsp Kosher salt, 12 ounces Gruyere cheese, 8 ounces extra-sharp cheddar, 1 tsp freshly ground black pepper, ½ tsp ground nutmeg
Assemble the casserole: Return the drained pasta to the large pot. Add the cheese sauce and cooked mushrooms (except any reserved for garnish), stirring until evenly coated. Transfer to a greased 9×13-inch baking dish. Sprinkle the reserved Gruyère evenly over the top.
Make the breadcrumb topping: Pulse the garlic, parsley, and bread in a food processor until coarse crumbs form. Sprinkle evenly over the cheese, then arrange the reserved mushroom slices on top if using.
2 garlic cloves, 2 Tbsp fresh parsley, 3 slices fresh white bread
Bake: Bake uncovered for 35–45 minutes, until the sauce is bubbling around the edges and the topping is golden brown. If the breadcrumbs begin browning too quickly, loosely tent the dish with aluminum foil for the remaining baking time.
Rest and serve: Let the macaroni and cheese rest for 5–10 minutes before serving so the sauce can thicken slightly.