Place the potatoes in a large pot and fill with enough water to cover. Bring to a boil, then reduce heat to medium and cook until tender enough to pierce with a fork, about 15 minutes. Drain and cool slightly. Remove the peels and cut into cubes. Set aside to cool completely.
In a large bowl, mix together the onion, salad dressing and mustard. Stir in the hard-cooked eggs and cooled potatoes. Try not to mash the potatoes very much. Season with salt and pepper.
