1. Cover potatoes with water by 1 inch in a medium saucepan and bring to a boil. Cook until tender, about 10 minutes; drain.

2. Place eggs in a small saucepan; add water to cover by 1 inch. Bring to a rolling boil. Remove pan from heat; cover, and let stand 7 minutes. Drain, and run eggs under cold water to cool.

3. Peel and halve eggs lengthwise. Remove yolks and transfer to a bowl; mash with a fork. Mix in mayonnaise, mustard, vinegar, paprika, and 2 tablespoons water; press through a sieve. Roughly chop egg whites; add to yolk mixture along with potatoes, celery, and onion. Season with salt and pepper. Toss to combine; top with more celery and onion.