3 tablespoons butter
1 lb. (Spanish) onions, halved & thinly sliced
1 tablespoon cider vinegar
1+1/2 lbs. potatoes, peeled & quartered (I use russet or Yukon Gold, or a combination of both)
1/2 cup hot milk
salt & pepper, to taste
In a large skillet, melt butter over medium-low heat. Add onions, stirring to coat with butter. Cover & cook 10 minutes, or until onions soften and exude liquid. Uncover, increase heat to medium and cook 15-20 minutes longer, stirring frequently, until onions are browned and reduced to 3/4 cup. (Sometimes this takes longer--it really depends on how carmelized you want them.) Stir in vinegar and set aside.
Boil potatoes until tender. Drain potatoes, then return to pot & mash. Gradually stir in hot milk until potatoes reach the consistency you like. Stir in browned onions, salt & pepper. (I usually add more butter too!)