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. Whisk in flour and cook for 1 minute, stirring constantly, until the mixture forms a smooth roux and turns lightly golden.
04 - Gradually pour in the whole milk while whisking continuously to prevent lumps. Bring to a gentle simmer and cook for 4–5 minutes, whisking constantly, 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 completely 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 - 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 brown and crisp.
08 - Remove from oven and let rest for 2–3 minutes. Serve hot while the cheese sauce is still creamy and the topping is crisp.