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Preheat oven to 350 degrees F. In a large pot of boiling, salted water cook the pasta to al dente.
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While the pasta is cooking, in a separate pot, melt the butter. Whisk in the flour and mustard and keep it moving for about five minutes making sure it's free of lumps.
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Stir in the milk, onion, bay leaf, and paprika. Simmer for ten minutes and remove the bay leaf.
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Temper in the egg. Stir in 3/4 of the cheese. Season with salt and pepper.
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Fold the macaroni into the mix and pour into a 2-quart casserole dish. Top with remaining cheese.
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For the topping, melt the butter in a sauté pan and toss the bread crumbs to coat. Top the macaroni with the bread crumbs. Bake for 30 minutes.
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Remove from oven and rest for five minutes before serving.