Preheat over to 425 degrees F (220 degrees C)
Line small rimmed baking sheet with foil or spray with nonstick cooking spray.
In bowl, toss onions with butter and vinegar until coated. Spread out onto baking sheet; roast in oven, stirring occasionally, until golden and soft, about 20 mins.
Meanwhile, peel and quarter potatoes. In large saucepan, cover potatoes with cold water; bring to boil. Cook for about 20 mins or until fork tender. Drain; return to low heat for 2 min to dry; shaking pan occasionally.
Set aside 1/3 of the roasted onions. Mash potatoes while gradually adding hot milk. Stir chopped onions, salt and paper into potatoes. Serve potatoes sprinkled the reserved potatoes.