3 tbsp. flour
1 egg
2 tbsp. butter
1 tsp. salt
1 tbsp. sugar
2 tbsp. mustard
1 c. milk
Dash of pepper
1/2 tsp. vinegar

Combine all ingredients except vinegar in saucepan. Cook over medium heat until mixture thickens, stirring constantly. Remove from heat. Stir in vinegar. Pour over warm potatoes.