Place potato chunks in a large pot and cover with cold water by about 2 inches. Add 1 tablespoon kosher salt.
Bring to a boil over high heat, then reduce to a simmer. Cook until potatoes are tender when pierced with a fork but not falling apart, about 12-15 minutes.
While potatoes cook, make the dressing: In a large mixing bowl, whisk together mayonnaise, sour cream, Dijon mustard, apple cider vinegar, sugar, black pepper, and paprika until smooth.
Drain potatoes well in a colander and let them steam dry for 5 minutes. They should still be warm but not hot.
Add warm potatoes to the dressing bowl and gently fold with a rubber spatula until evenly coated. The warmth helps the potatoes absorb the dressing.
Let the potato mixture cool to room temperature, about 30 minutes, stirring occasionally.
Once cooled, add chopped celery, red onion, green onions, and parsley. Fold gently to combine.
Taste and add salt if needed. Remember the boiling water was salted, so start with a little.
Cover and refrigerate for at least 2 hours to allow flavors to meld. Overnight is even better.
Before serving, give it one final gentle stir and adjust seasoning if desired. Serve chilled.
