Begin by preheating your oven to 400°F (200°C) and cooking 12 oz (350g) of elbow macaroni in boiling salted water for 8–10 minutes until just tender. While the pasta cooks, prepare the béchamel sauce—the creamy foundation of this dish.
In a large saucepan, melt 3 tbsp butter over medium heat, then add ⅓ cup all-purpose flour and cook, stirring constantly, for 1 minute to create a roux. Remove the pan from the heat and gradually blend in 3¾ cups milk, whisking to avoid lumps.
Return to the heat and bring to a boil, stirring constantly until the mixture thickens, then simmer for about 5 minutes while continuing to stir occasionally.
Once the sauce reaches the desired consistency, remove it from the heat and stir in 2 tsp Dijon mustard along with the three cheeses: 6 oz (175g) smoked Cheddar, 2 oz (60g) light mozzarella, and 2 oz (60g) of the aged Cheddar cheese.
The remaining 1 oz (30g) of aged Cheddar is reserved for the topping. Fold in the drained macaroni and season generously with salt and black pepper to taste, ensuring the pasta is evenly coated with the creamy cheese sauce.
Transfer the macaroni mixture to a lightly buttered large shallow ovenproof dish, spreading it evenly. Combine the ½ cup fresh white bread crumbs with the reserved 1 oz (30g) aged Cheddar cheese and sprinkle this mixture over the top of the macaroni.
Bake for 15–20 minutes until the surface is golden brown and the sauce is bubbling around the edges, indicating it’s fully heated through and ready to serve.