Instructions: Melt butter in a large wide-bottom pot or large, deep skillet over medium-low heat. Add onions; stir to coat. Cover and cook 10 minutes or until onions are softened and exude liquid. Uncover, increase heat to medium-high and cook about 20 minutes longer, stirring often until onions are very soft, golden brown and reduced to 2 3/4 cups. Stir in vinegar; remove to a bowl. Meanwhile, in another large pot, cook potatoes in water to cover 15-20 minutes until tender when pierced. Reserve 1/2 cup cooking water; drain potatoes. Heat milk in a pan or microwave until hot. Return hot potatoes to pot. Mash with a handheld mixer on medium speed or potato masher until smooth. Gradually beat or mash in hot milk, salt and pepper until potatoes are smooth and fluffy, gradually adding reserved cooking water if needed. Stir in onions, reserving some for garnish.

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