Stovetop Macaroni & Cheese

Inspired by Ina Garten's Cauliflower Gratin
Ingredients
1 lb pkg of macaroni (elbows or something similar in size)
Kosher salt
4 tablespoons (1/2 stick) unsalted butter, divided
1 tablespoons potato flour
2 cups hot milk
1/2 large sweet onion, chopped, sautéed until caramelized
1/2 teaspoon freshly ground black pepper
3/4 cup freshly grated smoked Gruyere, divided
3/4 cup freshly grated Parmesan

Directions
Preheat the oven to 375 degrees F.
Cook the pasta according to package directions. Drain.
Meanwhile, melt 2 tablespoons of the butter in a medium saucepan over low heat. Add the flour, stirring constantly with a wooden spoon for 2 minutes. Pour the hot milk into the butter-flour mixture and stir until it comes to a boil. Boil, whisking constantly, for 1 minute, or until thickened. Off the heat, add 1 teaspoon of salt, the pepper, nutmeg, sautéed onion, Gruyere, and Parmesan.

Toss pasta in cheese sauce and serve.