Add flour to a 1-quart saucepan. Cook over medium heat about 1 minute, stirring constantly. Add butter and continue cooking, stirring constantly to form a roux, about 2 to 4 minutes. Remove from heat and whisk in milk. return to stove, reduce heat to low-medium and cook, stirring constantly until sauce starts to thicken, about 2 to 3 minutes. Remove from heat and add cheese, stirring until melted in and thoroughly blended.