Place potatoes in a large pot and add enough cold water to cover by 1 inch. Bring to a boil over high heat, reduce to medium-low and simmer until potatoes are easily pierced with a fork, 30 minutes. Drain; slightly cool.
Cut potatoes in half and place in a large bowl. Add onion and celery. In a separate bowl, mix mustard with vinegar, mayonnaise, relish, salt and pepper. Pour over potatoes and gently stir to mix. Serve at room temperature or chilled.
I made this exactly as the recipe called and there was way too much mustard. I think the addition of other veggies would have helped. Also, I would cut the potatoes into smaller pieces instead of in half.
Yum! I tossed in ~ 1/2 cup of frozen peas to the potatoes while they were draining. Next time I plan to dice a small onion since hubby felt there were too many sliced onions. I didn't have whole-grain mustard on hand & used Grey Poupon. A wonderful addition to my file of recipe favorites!