Place the potatoes in a large pot, cover with water, and add a pinch of salt. Bring to a boil and cook until they are tender enough to be pierced with a knife, then drain. Once the potatoes are cool enough to handle, peel them and cut into cubes.
In a large mixing bowl, combine the cubed potatoes with the diced celery and chopped green onions.
In a separate bowl, whisk together the sour cream, mayonnaise, Dijon mustard, sugar, cayenne, salt, and black pepper to create the dressing.
Pour the dressing over the potato mixture and toss until everything is well coated. Adjust the seasoning with more salt and pepper if needed.