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 thoroughly in a colander and set aside.
03 - In a medium saucepan over medium heat, melt 2 tablespoons of butter. Sprinkle in the flour and whisk constantly for 1 to 2 minutes until the mixture turns a pale golden color and gives off a lightly toasted aroma.
04 - Pour in the milk in a slow, steady stream while whisking vigorously to prevent lumps from forming. Continue cooking and stirring for 3 to 5 minutes until the sauce thickens enough to coat the back of a spoon.
05 - Remove the saucepan from heat. Add the grated cheddar, Gruyère, and Dijon mustard, stirring until all the cheese has melted into a smooth, velvety sauce. Season with salt and 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 into an even layer.
07 - In a small bowl, combine the breadcrumbs with the remaining grated cheddar and drizzle in the melted butter if using. Toss until the crumbs are evenly moistened, then scatter 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 deeply golden and the sauce is bubbling around the edges.
09 - Remove from the oven and let stand for 5 minutes to allow the sauce to settle before serving.