01 - Preheat oven to 350°F if baking with the 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. Whisk in flour and cook for 1 minute, stirring continuously to form a smooth roux.
04 - Gradually whisk in the whole milk. Simmer, whisking constantly, until the sauce thickens and coats the back of a spoon, about 4–5 minutes.
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, ensuring every piece is evenly coated.
07 - For a baked version, transfer the mixture to a greased baking dish. Combine breadcrumbs, melted butter, and grated Parmesan in a small bowl, then sprinkle evenly over the top. Bake for 15–20 minutes until the topping is golden and crispy.
08 - Serve immediately while hot and bubbling.