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Cook pasta until al-dente. Melt the butter in a saucepan over medium heat. Add flour, and cook until the mixture is pale and yellow. Add mustard, paprika and onion and cook 1-2 minutes. Add milk, and cook at medium-high until slightly thickened, about 7-8 minutes. Remove from the heat.
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In a small bowl, beat the egg, then drizzle 1/2c of the milk mixture, whisking constantly. Then, add the egg mixture into to the remaining milk mixture. Stir in cheese, salt and pepper. Fold in the macaroni, then place in a 4-quart casserole dish.
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Sprinkle with the remaining cheese. Toss the breadcrumbs with the butter and sprinkle evenly over the top.
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Bake at 350 for 30 minutes.