01 - Preheat oven to 350°F if preparing the baked version with breadcrumb topping.
02 - Bring a large pot of salted water to a boil. Cook elbow macaroni according to package directions until al dente. Drain thoroughly and set aside.
03 - In a saucepan, melt butter over medium heat. Add flour and whisk continuously for 1 minute to form a smooth roux.
04 - Gradually pour in the whole milk while whisking constantly to prevent lumps. Simmer for 4 to 5 minutes, whisking frequently, until the sauce thickens enough to coat the back of a spoon.
05 - Reduce heat to low. Add both sharp and mild cheddar cheeses along with Dijon mustard. Stir until the cheeses are fully melted and the sauce is smooth. Season with salt and black pepper to taste.
06 - Fold the cooked macaroni into the cheese sauce, stirring gently until every piece is evenly coated.
07 - For a baked finish, transfer the mixture to a greased baking dish. Combine breadcrumbs, melted butter, and grated Parmesan in a small bowl, then sprinkle evenly over the surface. Bake for 15 to 20 minutes until the topping is golden and crisp.
08 - Remove from oven and let rest for 2 to 3 minutes. Serve hot while the cheese sauce is still creamy.