01 - In a pot of boiling salted water, cook the diced carrots, potatoes, and celery root until tender, about 20 minutes. Add frozen peas in the last 5 minutes of cooking. Drain and let cool.
02 - In a large bowl, combine the cooled vegetables and diced pickle.
03 - In a separate bowl, mix together the mayonnaise, sour cream, mustard, salt, and pepper.
04 - Pour the dressing over the vegetable mixture and stir until well combined.
05 - Refrigerate for at least 1 hour before serving to allow flavors to meld.