Place potatoes and 2 tbsp of kosher salt in a medium pot and cover with water. Bring to a boil over high heat, then reduce to a simmer and cook potatoes until easily pierced with a knife, about 12-15 minutes. Drain potatoes into a colander and place colander over the empty pot. Cover with a kitchen towel and let steam for 15-20 minutes.
While the potatoes are steaming, combine sour cream, mayonnaise, both mustards, relish, vinegar, worcestershire, dill, salt and pepper in a large bowl.
Add red onion, celery and green onion and combine.
Add steamed potatoes and gently fold until the mixture coats the potatoes. Taste for seasonings. Cover and refrigerate for a few hours to blend flavors. Taste before serving, adjust seasonings as desired.