1. Preheat the broiler. In a medium saucepan of boiling salted water, cook the macaroni until al dente; drain well.
2. Meanwhile, in another medium saucepan, melt the butter over low heat. Whisk in the flour until a paste forms. Gradually whisk in the milk until smooth. Bring the sauce to a boil over moderately high heat, whisking, until thickened. Off the heat, stir in the mixed shredded cheeses until melted. Season with salt and pepper.
3. Add the macaroni to the hot cheese sauce and stir to coat; transfer to a 9-by-13-inch glass or ceramic baking dish and sprinkle on the Parmigiano-Reggiano. Broil 4 inches from the heat for 4 minutes, until richly browned. Let rest for 5 minutes before serving.