Discard the remaining pasta cooking water from the large pot used to cook the pasta to use for the cheese sauce. Over medium heat, add the shallot, white wine, and sherry. Cook for several minutes until there is only about 2 teaspoons of liquid left.
½ small shallot, ¼ cup dry white wine, 1 Tablespoon dry sherry
Next, add the butter and stir until melted. While whisking, slowly add the flour. Bring all of this together, making a roux. You want a blonde roux so keep the heat down and just cook until the smell of raw flour is gone but the mixture isn’t brown. This will take just a few minutes.
3 Tablespoons unsalted butter, 3 Tablespoons all-purpose flour
Next, slowly add the cream while whisking constantly and once combined whisk in the mustard powder, creating a béchamel sauce.
1 cup + 2 Tablespoons heavy cream, 1 teaspoon mustard powder
Finally, stir in the fontina, gruyere and mascarpone cheeses, thyme, salt and pepper. Continue to stir until the cheeses are completely melted and you have a nice smooth sauce. If it seems too thick, add a little water to thin it out. You want the sauce to be thick and creamy but still saucy enough to coat the pasta and seafood.
3 Tablespoons mascarpone cheese, ¼ cup fontina cheese, ¼ cup gruyere cheese, 2 teaspoons fresh thyme leaves, 1 teaspoon kosher salt, Pinch black pepper