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Cook pasta, slightly tougher than al dente, remove from pot, set aside.
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Preheat oven to 325.
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Melt butter.
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Combine butter, flour, and mustard. Whisk to incorporate into butter over medium low heat. Be careful not to brown the flour and whisk enough to eliminate lumps.
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Add milk, half and half, paprika, onion, salt, and bay leaf.
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Bring just to a boil and then reduce to a simmer. Simmer approximately 8 minutes.
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Remove bay leaf.
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Temper the egg (http://www.ehow.com/how_4731920_temper-an-egg.html).
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Incorporate cheese, one third at a time, ensuring it is all melted before adding the next wave. Reserve about 2 cups for on top.
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Carefully fold in macaroni.
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Move to casserole.
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Top with remaining cheese.
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Combine dry topping ingredients.
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Melt butter in small skillet.
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Toss dry mixture in skillet until coated.
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Top casserole with topping.
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Bake 45 minutes.
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Allow to rest for ten minutes.