Mix potatoes and peas together and season to taste with salt and pepper. Stir in melted butter with a fork. Add beets, stirring only until barely mixed to preserve the color appeal of the salad (beets will bleed if stirred too much). Refrigerate and serve in chilled lettuce cups with Cooked Salad Dressing.
Cooked Salad Dressing
Scald milk in a double boiler. Mix mustard, sugar, flour, and salt thoroughly with a spoon. Stir a little of the scalded milk into this mixture, then pour back into the double boiler, stirring constantly while mixture thickens. Cook for 10 minutes. Stir in beaten eggs and cook for no more than 2 or 3 minutes. Remove from heat. Add hot vinegar and butter. Strain, cool, and store in refrigerator. Makes approximately 2-½ cups.