Heat margarine in 1 1/2 quart saucepan over low heat until melted. Stir in flour, salt and pepper. Cook over low heat stirring constantly until mixture is smooth and bubbly. Remove from heat. Stir in milk. Heat to boiling stirring constantly. Boil and stir 1 minute.
To add cheese-
Stir in dry mustard with the flour. After boiling and stirring sauce 1 minute, stir in cheddar cheese until melted.
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