Scrub potatoes clean but don’t peel them. Boil in salted water until tender but still firm, about 20-25 minutes. Drain and let cool slightly.
In a small saucepan, combine vinegar, water, sugar, salt, pepper, and mustard. Bring to a boil, then reduce heat and simmer for 5 minutes. Let cool completely.
Cut cooled potatoes into bite-sized pieces. Add chopped onion to the potatoes.
Slowly whisk the oil into the cooled vinegar mixture until emulsified. Pour the dressing over the potato and onion mixture.
Stir gently to combine. Serve immediately or chill for later. Garnish with fresh chives.
