Preheat the oven to 375°F. Butter an 8-inch square (1 1/2-quart) baking dish or pan (or six 8-ounce ramekins). Set aside.
Fill a 4- to 5-quart pot about three-quarters full with water and add 1 tablespoon of the salt. Bring to a boil and add the pasta. Cook, stirring once or twice, until tender but firm, about 4 minutes. Drain, and reserve the pot.
While the pasta is cooking, in a medium skillet, melt 2 tablespoons of the butter over medium heat. Turn off the heat and add the breadcrumbs and Parmigiano-Reggiano. Stir until mixed well. Set aside.
Using the same pot you used to cook the pasta, melt the remaining 3 tablespoons butter over medium heat. Add the onion and cook, stirring occasionally, until soft and translucent, about 5 minutes. Slowly whisk in the flour and stir constantly until the onion is coated with the flour, 30 to 45 seconds. Continue stirring for about 2 minutes more, or until the mixture starts to darken slightly and smell a bit nutty.
Slowly whisk in the milk, cream and the remaining 1 teaspoon salt and cook until the mixture is just beginning to thicken and bubble around the edges, about 5 to 7 minutes. It should be similar in texture to cake batter. If it's soupy, continue cooking until it thickens. Add 1 1/2 cups of the cheddar, the Gruyère, mustard powder, cayenne and nutmeg and stir until the cheeses have melted and the sauce is smooth but not too runny. Again, it should be similar in texture to cake batter. If it's soupy, continue cooking, stirring constantly, until it thickens.
Add the pasta and stir to combine. Pour into the prepared baking dish. Sprinkle with the remaining 1/2 cup of cheddar and top with the breadcrumb mixture.
Place the dish on a rimmed baking sheet and bake until bubbling and golden brown, about 30 minutes. Let cool for 15 minutes before serving.
2. I'm melting!
Because mac & cheese is all about melted goodness, it's essential to use cheeses that submit to their full glory when exposed to heat. In other words, USE GOOD MELTERS.
For some of the best melting cheeses, head for the hills - preferably the Swiss Alps! It's there where you'll find the unparalleled Gruyère (one of the best-tasting melting cheeses in the world), Emmentaler (the original "Swiss" cheese) and Appenzeller, among others. Hop over those hills into France's Jura region and grab some Comté for an equally memorable melting cheese experience.
And from the rolling hills of Wisconsin, Pleasant Ridge Reserve and just about any Wisconsin cheddar will make any mac & cheese sing, as will the cheeses from the farms and forests in Vermont all the way to the seaside farms in California, Oregon and Washington. So, now that you know the fundamentals, it's time to put them into practice.
3. Thanksgiving mac attack
Ravioli with butternut squash, brown butter and sage has become a classic fall pasta combination in many upscale restaurants. Why not take it down-home and fold that delectable flavor combination into mac & cheese? This recipe is a true winner and a dish that your friends and family will love you for, whether you serve it at Thanksgiving dinner or even in spring or summer.
Butternut Squash, Gruyère and Brown Butter Mac & Cheese
1 Tbsp, plus 1/2 tsp kosher salt
8 oz mini farfalle pasta (or you can use small elbow macaroni)
5 Tbsp salted butter
2 cups coarse, fresh breadcrumbs (preferably homemade)
4 oz pecorino Romano or Parmesan cheese, finely grated (about 2 cups)
1 Tbsp finely chopped fresh sage leaves, plus 12 whole leaves
1 pound butternut squash, peeled, seeded and cut into 1/2-inch chunks
1 1/2 cups whole or reduced-fat milk
1 1/2 cups heavy cream