Meanwhile, in 2-quart saucepan, melt butter over low heat. Stir in flour; cook 1 minute, stirring constantly with whisk. Remove from heat; stir in milk. Heat to simmering; cook 1 minute, stirring constantly, until thickened. Stir in salt, pepper, nutmeg, mustard and pumpkin. Add 1 3/4 cups of the cheese. Cook, stirring occasionally, until cheese is melted.