Combine the milk, garlic, bay leaf, thyme, and peppercorns in a 2-quart saucepan over medium heat. Cook until the milk is hot and small bubbles appear around the edges of the pan, about 8 minutes. Remove from heat and allow aromatic ingredients to steep in the milk for 10 minutes. Pour the milk through a fine mesh strainer; discard aromatics. Wash out the pan and return it to medium heat.
Melt the butter in the pan over medium heat. Whisk in flour and cook, stirring constantly for 1 minute. Gradually whisk in the warm milk and cook over medium low heat, stirring frequently, until the sauce has thickened, about 5 minutes. Monitor the sauce carefully and reduce heat if sauce begins to scorch on bottom of pan. Reduce heat to low, whisk in the mustard, nutmeg, cayenne and half of the grated cheeses. Season with salt and pepper and set aside.
Preheat the broiler and adjust the oven rack so that it is 6 inches below the broiling element. Spray a 9×13-inch baking dish with cooking spray.
Bring a large pot of water to a boil. Add 1 tablespoon of salt and the pasta to the pot and boil until al dente, about 8 minutes. Drain and return pasta to pot. Stir in the cheese sauce.
Pour the pasta-sauce mixture into the prepared baking dish. Sprinkle with the remaining cheese and broil until the cheese is bubbly and crisp in places, 5 minutes. Serve immediately.
