Bring a pot of salted water to a boil, add potatoes, and cook until fork-tender, 12 to 15 minutes. Drain and let cool slightly, keeping them warm.

In a large bowl, whisk mayonnaise, sour cream, pickle juice, mustard, salt, and pepper. Stir in pickles, celery, red onion, and half the dill.

Cut warm potatoes into bite-sized chunks, add to the dressing, and fold gently to coat. Let sit at room temperature for 10 minutes.

Fold in remaining fresh dill, transfer to a serving bowl, garnish with extra pickle slices, and serve cool or at room temperature.