Place potatoes in a 6-qt. stockpot; add water to cover. Add salt; bring to a boil. Reduce heat; cook, uncovered, until potatoes are tender, 12-15 minutes., Meanwhile, in a large skillet, cook bacon over medium heat until crisp, stirring occasionally. Remove with a slotted spoon and drain on paper towels; reserve 4 tablespoons bacon drippings. Cook and stir onion in reserved drippings until browned, 6-8 minutes., Reserve 1/4 cup cooked bacon for topping. Add onion, drippings, celery and remaining bacon to potatoes., In a small bowl, mix mayonnaise, mustard and seasonings. Pour over potato mixture; toss to coat. Refrigerate, covered, until chilled, about 1 hour. Just before serving, sprinkle with reserved bacon.