Set a steamer basket in a saucepan with 1 inch simmering water. Add potatoes, cover, and steam until potatoes are tender, 15 to 18 minutes. Transfer potatoes to a medium bowl and add olive oil, vinegar, and mustard; toss to combine. Using the large holes on a box grater, grate egg into potatoes. Add onion, season with salt and pepper, and toss to combine.
Good with pork chops, salmon burgers, or chicken cutlets.