Place diced potatoes in a large pot, cover with water, add 1 tsp salt, and bring to a boil.
Reduce heat and simmer until potatoes are fork-tender, about 12–15 minutes. Drain and cool slightly.
Prepare hard-boiled eggs: boil for 10 minutes, cool in ice water, peel, and chop.
In a medium bowl, whisk together mayonnaise, Dijon mustard, apple cider vinegar, sugar, salt, and black pepper until smooth.
Combine potatoes, eggs, celery, and red onion in a large mixing bowl. Pour dressing over and fold gently to coat.
Refrigerate at least 1 hour before serving. Garnish with paprika and parsley.
