Bring a large pot of salted water to a boil. Add the conchiglie and cook until tender but not soft (al dente). Drain and toss with 1 tablespoon of the truffle oil. Set aside.

Preheat the oven to 350°F and generously butter a 2-1/2 or 3-quart baking dish.

Melt 1 tablespoon of the butter in a large skillet over medium heat. Add the garlic and sauté until fragrant, 1 minute. Add the sliced mushrooms and continue cooking until nicely browned, 6 to 7 minutes. Season with truffle salt and a few grinds of black pepper. Remove from the heat and set aside.

Heat the milk in the microwave or a separate saucepan until very warm, but not scalded. Set aside.

Melt the remaining 4 tablespoons of butter in a large, heavy saucepan over medium heat. Sprinkle in the flour and blend with a whisk until well combined. Continue cooking, whisking constantly, until the mixture develops a pale golden color, 3 to 4 minutes. Gradually add the warmed milk, whisking continually until well blended.

Continue cooking, whisking frequently, until the sauce is smooth and thick enough to coat the back of a spoon, 7 to 8 minutes. Scrape down the sides of the pan with a spatula once or twice during cooking to ensure there are no lumps in your sauce. Season to taste with truffle salt.

Reduce the heat to low, add the fontina and stir until completely melted. Add the mozzarella cubes, a few at a time, stirring until melted after each addition. Add the reserved mushroom mixture, stir until smooth, then add the pasta and combine well.

Transfer the mixture to the prepared baking dish and bake uncovered for 15 minutes.

Remove the macaroni and cheese from the oven, top with the crumb mixture and drizzle with the remaining tablespoon of truffle oil. Return to the oven for an additional 5 minutes.

Allow the macaroni and cheese to rest for 5 to 7 minutes before serving.