In a medium saucepan, boil the potatoes until tender. Peel and slice about 1/4 inch thick and set aside.
Fry bacon. Set aside on paper towel to cool. Do not discard the drippings.
Add flour to the drippings in skillet on medium heat, stirring to make a rue. Slowly add the water and stir until the until the mixture is the consistency of gravy.
Reduce heat to simmer. Add mustard and vinegar. Continue cooking and stirring until the mixture is smooth. Remove from heat and allow to cool to room temperature.
While the mixture cools, crumble the bacon and set aside. Coarsley chop the hard boiled eggs and set aside.
When the dressing mixture is cooled, add the mayonnaise and stir until smooth. This mixture can then be reheated until warm.
Place the potatoes in a large bowl, add the chopped onion and egg and crumbled bacon. Pour the dressing mixture over the potatoes and gently mix together. Too much muscle will give you mashed potatoes.