3 tbsp. butter
3 tbsp. flour
1 tsp. salt
3/4 tsp. dry mustard
1/4 tsp. pepper
1 med. chopped onion
1 1/2 c. milk
3/4 c. mayonnaise
6 c. diced potatoes, cooked

Saute onions and set aside. Melt butter in saucepan; blend in flour, salt, mustard and pepper. Slowly stir in milk and cook until mixture thickens and boils 1 minute. Blend in mayonnaise, then add potatoes and sauteed onions. Pour into casserole, top with buttered crumbs. Bake in 350 degree oven 45 minutes.