01 - Preheat the oven to 400°F. Lightly butter a medium baking dish and set aside.
02 - Bring a large pot of generously salted water to a rolling boil. Add the macaroni and cook according to package directions until al dente. Drain through a colander and set aside.
03 - In a medium saucepan over medium heat, melt 2 tablespoons of butter. Sprinkle in the flour and whisk continuously for 1 to 2 minutes until the mixture turns a pale golden color and gives off a lightly toasted aroma.
04 - Slowly pour in the whole milk while whisking vigorously to prevent lumps from forming. Continue cooking and stirring over medium heat for 3 to 5 minutes until the sauce thickens enough to coat the back of a spoon.
05 - Remove the saucepan from the heat. Add the grated cheddar, Gruyère, and Dijon mustard, stirring until the cheeses are fully melted and the sauce is smooth. Season generously with salt and black pepper to taste.
06 - Fold the drained macaroni into the cheese sauce, stirring gently until every piece is evenly coated. Transfer the mixture to the prepared baking dish, spreading it out in an even layer.
07 - In a small mixing bowl, combine the breadcrumbs, remaining grated cheddar, and melted butter. Toss until the breadcrumbs are evenly moistened, then sprinkle the mixture uniformly over the macaroni.
08 - Place the dish in the center of the oven and bake for 15 to 20 minutes, until the topping is crisp and deeply golden and the sauce is bubbling around the edges.
09 - Remove from the oven and let the dish rest for 5 minutes to allow the sauce to set slightly before serving.