Heat a skillet and then add the olive oil.
Once the oil is hot, add the mushrooms, sherry and salt and pepper to taste and cook over medium-high heat until for about 3-4 minutes, or until the mushrooms have given up all their liquid and the pan is almost dry.
Stir in the fresh thyme. Set the mushrooms aside.
In the same pan, heat the butter until bubbly.
Add the flour and whisk and cook until golden, about 3 minutes.
Add the 3 cups of milk and stir until boiling and thickened. Season with salt and pepper.
Off the heat, stir in the goat cheese, cheddar and fontina, a bit at a time. You might need to put the mixture back over low heat, but don’t let it boil.
Stir in the truffle oil and the reserved mushrooms.
Cook the pasta according to package directions to al dente, drain well and stir into the cheese-mushroom mixture.
Taste, and correct seasonings.
Put the pasta in a baking dish.
Mix the panko and Parmesan and sprinkle evenly over the top of the casserole.
Bake the mac and cheese in the upper third of a 400°F oven until the cheese is bubbling and the topping is a deep golden brown.