In saucepan combine butter and cornstarch. Add milk, sugar, vinegar, salt, celery seed, mustard and cook until thick. In a separate bowl, beat an egg well with a fork and gradually add 1/2 of the hot mixture to the egg. Then return to remaining hot mixture and cook a few more minutes. Remove from heat. Add onion and mayonnaise. Add 1 or 2 hard
boiled sliced eggs and sweet relish to potatoes. You might have a add a little more mayonnaise.