Peel and cook potatoes, dice into 1/2" (or slightly larger, but not smaller) uniform pieces.
Chop celery stocks, dice onion, and thinly slice the green portion of scallions.
In a large bowl, combine potatoes, celery, onion, scallions, 3 tablespoons of parsley (reserving the remainder for garnish), rice vinegar, mayo, Dijon mustard, pepper, and salt.
Mix well and garnish with remaining parsley.
Refrigerate until it's time to serve.