Step 1Heat the oven to 400 degrees.
Step 2In a large, heavy-bottomed saucepan, melt the butter over medium heat. Add the onion and garlic, and cook to soften, 5 minutes.
Step 3Stir in the flour to form the roux. Continue cooking, stirring constantly, until the roux goes from a light white color to a blond golden color, about 5 minutes. Add the bay leaf, peppercorns, coriander seeds, yellow mustard seeds and the nutmeg, and cook, stirring, for 2 minutes to distribute the flavors.
Step 4Whisk in the milk a little at a time. Increase the heat and bring to a simmer, stirring constantly. Reduce the heat to low and cook for 30 minutes to cook out the flour flavor, stirring occasionally to make sure the flour doesn't sink to the bottom of the pan and burn.
Step 5Remove the pan from heat and strain into a new pot, discarding the spices. Add the American and half of the Tallegio cheeses, stirring until the cheese is melted and incorporated. Taste the sauce, adding 1 1/2 teaspoons salt, or to taste.
Step 6Add the pasta and the remaining Tallegio cheese, stirring to coat the pasta completely. The mixture at this point will seem a bit wet but will thicken as it bakes.
Step 7Pour the mixture into a 13-by-9-inch or similar large baking dish and bake until the mixture is thickened and bubbly, about 20 minutes. The mac 'n' cheese should have a nice crust on top. If not, place it under the broiler to darken a little.
Step 8Remove from heat and cool slightly before serving.