We make no judgment if you occasionally reach for the “instant” stuff. But try our made-from-scratch version—you’ll be hooked. We’ve added onion and cayenne for another layer of flavor, and they help keep this dish from turning into a gloppy mound of melted cheese (not that there’s anything wrong with that).
Heat oven to 350º. Butter a 13x9-inch casserole pan.
Bring a large pot of salted water to a boil. Cook pasta about 2 minutes less than package instructions indicate, so that it’s still slightly crunchy. Drain and set aside.
In a large saucepan over medium heat, melt butter and cook onion gently until softened, about 5 minutes.
Reduce heat to medium-low. Whisk in flour and cayenne to form a thick paste; cook, whisking continuously, about 2 minutes.
Whisk in milk until smooth; cook, while occasionally whisking, until thickened and bubbly. Whisk in heavy cream. Whisk in cheeses. Season to taste with salt and pepper. Add noodles and stir well to coat.
Pour into buttered baking dish. Sprinkle with breadcrumbs. Bake 20 minutes or until top is lightly browned. Let rest 5 minutes before serving.