Boil cubed potatoes in salted water for 10–15 minutes until fork-tender, then drain and cool slightly to maintain their structure.
Whisk together mayonnaise, sour cream, mustard, vinegar, salt, and pepper until smooth and well combined.
Add chopped celery, onion, parsley, and eggs into the dressing and mix evenly to distribute flavor.
Fold warm potatoes into the dressing gently to coat without breaking them.
Chill the salad for at least 30 minutes to allow flavors to develop and improve texture.
Stir, taste, and adjust seasoning before serving to ensure the best flavor balance.