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Preheat your oven to 375 degrees.
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Cook pasta according to package directions and set aside.
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Melt butter in a medium saucepan. Remove from heat.
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Stir in flour, salt, pepper, and mustard and stir until smooth.
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Now add the milk and stir constantly until boiling.
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Remove from heat and stir in TWO cups of cheese.
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Reduce heat to low and continue to stir until everything is combined.
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Stir in pasta and pour into a baking dish.
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Sprinkle top with remaining cheese.
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Bake 15-20 minutes or until the cheese on top is golden brown and the casserole is bubbly.