While pasta is cooking, in a separate saucepan, melt butter on medium heat. Once melted, add flour, mustard, pepper & salt. Stir until a paste. Slowly stir in milk, stirring constantly over medium heat.
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Continue stirring until thick, approximately 10 minutes. Remove from heat and stir in 4 1/2 cups cheese, stirring until melted. Pour cheese mixture into casserole, mixing throughout macaroni thoroughly.
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