01 - Preheat the oven to 350°F. Lightly grease a medium baking dish with a 1.5 quart capacity.
02 - Bring a large pot of salted water to a boil. Cook the macaroni until just al dente, about 1–2 minutes less than package directions. Drain and reserve.
03 - In a medium saucepan over medium heat, melt 2 tablespoons butter. Sprinkle in flour and whisk constantly for 1 minute until combined and bubbling.
04 - Gradually whisk in milk. Stir continuously and cook until the sauce thickens and coats the spoon, about 4–5 minutes.
05 - Remove saucepan from heat. Stir in grated cheddar, Gruyère or mozzarella, Dijon mustard, pepper, salt, and nutmeg. Mix until cheeses are melted and sauce is velvety.
06 - Add drained macaroni to the cheese sauce and mix until evenly coated. Transfer mixture to the prepared baking dish.
07 - In a bowl, toss panko breadcrumbs with melted butter and Parmesan. Sprinkle the mixture evenly over the macaroni.
08 - Bake in the oven for 20–25 minutes, until the topping is golden brown and sauce is bubbling.
09 - Allow to stand for 5 minutes before serving.