Bring a pot of salted water to a boil over high heat, add the potatoes then reduce heat to a simmer and cook until a fork easily slides in and out with little resistance, about 5 to 7 minutes.
Melt the butter in a nonstick pan on medium heat. Saute the onions and garlic until soft. Stir in the vinegar, mustard, and sugar. Bring to a simmer until the sugar dissolves completely.
Remove from the heat. Stir in the potatoes and parsley. Season with salt and pepper to taste before serving.