Position a rack about 4 inches from the broiler and heat the broiler on high.
Bring a large pot of well-salted water to a boil over high heat. Add the pasta and cook according to package directions until just tender. Drain well and set aside.
Meanwhile, melt the butter in a 12-inch ovenproof skillet (preferably cast iron) over medium heat. Whisk in the flour and continue whisking until well combined, about 15 seconds. Whisk in the milk and continue to cook, whisking constantly, until the mixture thickens, 1 to 2 minutes.
Add the Emmentaler, Gruyère, mustard, Worcestershire sauce, and thyme and whisk until the cheese is melted and the mixture is smooth, 2 minutes. Stir in the pasta to coat with the sauce. Off the heat, season to taste with salt and pepper. Sprinkle the Parmigiano-Reggiano evenly over the pasta.
Broil until the top is browned, 3 to 4 minutes, and serve.
To get your veggies in, serve with sweet, salty, and tangy Balsamic-Bacon Vinaigrette Sauce over steamed broccoli, Brussels sprouts, or broccolini.
nutrition information (per serving):
32, Fat Calories
290, Saturated Fat
36, Monounsaturated Fat
77, Polyunsaturated Fat
Photo: Scott Phillips