Cook potatoes in boiling water until tender; drain and cool.
Meanwhile, in a saucepan, combine sugar, flour, salt, mustard and pepper. Add vinegar and water; bring to a boil. Boil and stir for 2 minutes. Add a small amount to eggs; return all to the pan. Cook and stir for 1-1/2 to 2 minutes or until mixture is thickened and a thermometer reads 160°. Refrigerate until cooled.
Peel potatoes if desired; slice and place in a large bowl. Add onions. Stir sour cream into dressing; pour over potato mixture and toss to coat.