Place dry pasta in a 4-quart pot and barely cover with cold water. Bring just to a boil over high heat, stirring occasionally, then test for doneness. You’re looking for al dente, that is the noodles should put up some resistance to the tooth but not so much that it gums up in your back teeth. Drain pasta in a colander but do not rinse.
Meanwhile, melt the butter in a 3-quart saucier or saucepan over medium heat. Whisk in the flour and cook until the mixture is pale blond, stirring occasionally, about 3 minutes. Whisk in the mustard, paprika and onion and cook another minute. Whisk in the milk and cook over medium-high heat, whisking often for 7 to 8 minutes, or until slightly thickened. Remove from the heat.
Lightly beat the egg in a small bowl then drizzle in about half a cup of the milk mixture, whisking constantly. Then, whisk the egg mixture into to the milk mixture. (This common maneuver called “tempering” prevents curdling of the egg by slowly increasing their temperature. Skip this and you’ll be making scrambled eggs.) Stir in the cheese, 1 teaspoon salt and the pepper. Fold in the macaroni then move to a 4-quart casserole or soufflé dish.
Sprinkle with the remaining cheese, then toss the breadcrumbs with the melted butter and distribute evenly over the top. Bake for 30 minutes, cool for 5 minutes, then serve to a grateful world.