I begin by bringing a large pot of salted water to a boil, then I cook the elbow macaroni just shy of al dente. It shouldn’t be mushy, since it will continue cooking in the oven. After draining, I drizzle a little butter or oil to prevent sticking.
In a saucepan, I melt butter over medium heat, then whisk in the flour to form a roux. It should cook a minute or two so it loses its raw flour taste. Slowly I whisk in milk (and cream, if using), mustard powder, nutmeg, salt, and pepper. As it heats, the sauce thickens; then I turn down the heat and stir in shredded cheeses in batches until melted smoothly.
I pour the pasta into the cheese sauce, stirring so every noodle is coated. Then I transfer it into a buttered baking dish. If using breadcrumbs, I mix them with a little melted butter and sprinkle evenly on top. The dish goes into a preheated oven; I bake until the top is golden and crusty, and sauce bubbles around the edges. Once out, I let it rest a few minutes before serving.