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preheat oven to 350 degrees
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cook elbow macaroni in boiling, salted water
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melt butter and saute grated onion 1-2 minutes until soft
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add flour, dry mustard, salt & pepper and saute 2 minutes until golden
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whisk in milk, slowly to make a smooth sauce
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heat sauce until it boils and thickens
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turn off the heat and stir in the cheddar cheese until melted
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drain the pasta and add to the sauce
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pour into a buttered 9x13 inch baking dish
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top with slices of american cheese
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bake for 20 minutes until hot and bubbly