Steamed potatoes are usually the base of this warm German potato salad, but here the roasted potatoes add another layer of flavor.
Preheat oven to 425 degrees F.
Toss potatoes in a large bowl with oil, 1/4 teaspoon salt and pepper. Transfer to a large rimmed baking sheet. Roast, stirring once, until tender and browned, 20 to 25 minutes.
Meanwhile, cook bacon in a medium skillet over medium heat, stirring occasionally, until crisp, 5 to 7 minutes. Transfer with a slotted spoon to a paper-towel-lined plate. Add onion, vinegar and mustard to the pan. Cook, stirring constantly and scraping up the browned bits, for 1 minute. Remove from heat.
Transfer the potatoes to a large bowl and drizzle with the dressing; stir to combine. Add celery and the bacon and toss to coat. Season with the remaining 1/4 teaspoon salt and more pepper to taste.