Preheat the oven to 400 degrees . Grease a 3-qt. casserole dish. Bring a large pot of salted water to a boil.
Meanwhile, in a large pan, bring the milk to a simmer over medium. In a large skillet, melt the butter over medium until it begins to bubble. Add the flour; cook, whisking, for 1 minute. Slowly whisk in the hot milk. Cook, whisking, until the sauce thickens, about 6 minutes. Stir in the Gruyere, fontina, Monterey jack, American cheese, 2 cups cheddar, mustard powder and nutmeg; season.
Cook the pasta until al dente; drain and rinse. Stir the pasta into the cheese sauce, then transfer to the prepared dish. Sprinkle with the remaining 1 1/2 cups cheddar and the Parmesan. Bake until browned, 30 to 40 minutes. Let cool 5 minutes before serving.