01 - Preheat 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 just al dente. Drain through a colander and set aside.
03 - In a medium saucepan over medium heat, melt the butter. Sprinkle in the flour and whisk constantly for 1 to 2 minutes until the mixture is smooth and lightly golden, forming a proper roux.
04 - Slowly pour in the milk while whisking vigorously to prevent any 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 heat. Add the grated cheddar, Gruyère, and Dijon mustard, stirring gently until all the cheese has melted into a smooth, velvety sauce. Season with salt and pepper to taste.
06 - Add the drained macaroni to the cheese sauce and fold together until every piece of pasta is evenly coated. Transfer the mixture to the prepared baking dish, spreading it out in an even layer.
07 - In a small bowl, combine the breadcrumbs with the remaining grated cheddar. Drizzle in the melted butter if using, and toss until the crumbs are evenly moistened. Sprinkle the topping uniformly over the macaroni.
08 - Place in the center of the oven and bake for 15 to 20 minutes until the topping is deeply golden brown 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.