For the mushrooms: Heat 1 tablespoon of the oil in a large saute pan over medium heat. Add the shallots and cook until soft, about 4 minutes. Add the garlic and cook for 30 seconds.
Increase the heat to high, add the remaining 2 tablespoons of oil and heat until it begins to shimmer. Add the mushrooms and cook until golden brown and soft, stirring occasionally, about 8 minutes. Add the wine and cook until reduced; season with salt and pepper. Set aside.
For the mac and cheese: Preheat the oven to 350 degrees F.
Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until al dente, about 9 minutes. Drain and rinse with cold water to stop cooking. Transfer to a large bowl.
Put the milk into a medium saucepan and bring to a simmer over low heat.
Heat the oil in a large ovenproof saute pan over medium heat until it begins to shimmer. Add the cauliflower, season with salt and pepper and toss to coat. Cook until slightly soft, about 5 minutes. Add 1 cup of water, cover loosely with foil and cook in the oven until soft.
To make the bechamel, melt the butter in a large saucepan over medium heat. Whisk in the flour and cook for 1 minute. Slowly whisk in 4 cups of the milk and cook, whisking constantly, until thickened and the flour taste has been cooked out, about 7 minutes, adding more milk if the sauce is too thick.
Remove the cauliflower from the oven and transfer to a blender with 1 cup of the milk and blend until smooth. Add the cauliflower puree to the bechamel and stir until combined. Slowly whisk in the fontina, Monterey Jack, asiago and Parmigiano-Reggiano until smooth, adding more hot milk if too thick. Mix in the nutmeg. Pour the sauce over the pasta, add the mushrooms and stir well to combine.
Butter a 10-inch baking dish, transfer the pasta mixture to it and bake until bubbly and the top is lightly golden brown, about 30 minutes.
For the breadcrumb topping: While the mac and cheese is baking, melt the butter in a large saute pan over medium heat. Add the anchovy paste and cook for 1 minute. Add the chiles and garlic and cook for 30 seconds. Add the breadcrumbs and cook until lightly toasted, about 5 minutes. Stir in the parsley and season with salt and pepper.
Serve the mac and cheese topped with some of the breadcrumbs and a drizzle of white truffle oil. Garnish with parsley leaves.