Preheat oven to 350 degrees F.

Cook the pasta in lightly salted water (important!) until al dente according to package directions. Drain and set aside (tossing once or twice to prevent clumping).

In the same pot (no need to wash, just wipe dry), melt the butter over medium heat. Add the flour, mustard (if using) and salt, and whisk to combine. Cook for 1 to 2 minutes, whisking constantly.

Gradually add the milk about one cup at a time, whisking to fully combine before adding more. Bring to a simmer, whisking constantly, and cook for 3 to 4 minutes until the sauce has thickened to the consistency of thick cream.

Off the heat, adding one handful at a time, whisk in 3 cups of the cheddar cheese and 1 cup of the gruyere cheese, mixing until the cheeses are fully melted and the sauce is creamy. (Reserve the remaining 1 ½ cups of cheese for assembling in the baking dish.)

Add the cooked macaroni noodles to the cheese sauce and gently mix until thoroughly and evenly combined.

To assemble, spread half of the macaroni and cheese in a 9X13-inch baking dish. Sprinkle evenly with the remaining cheese. Spread the rest of the macaroni evenly over the top.

In a small bowl, toss the bread crumbs and butter together until evenly mixed. Stir in the Parmesan cheese.

Sprinkle the topping evenly over the macaroni. Bake in the preheated oven for 20 to 25 minutes until the topping is golden and the macaroni and cheese is hot and bubbling. Serve immediately.