Cook cubed potatoes in a large pot covered by 2 inches of water.
Bring to a boil over high heat. Boil until tender, 15-20 minutes.
Drain potatoes.
While potatoes are cooking, whisk together vinegar, mustard, salt, pepper and sugar.
Then stir in olive oil.
Toss hot potatoes with dressing until coated, adding green onions and parsley.
Serve and enjoy!