Mashed Potatoes with Browned Onions

My favorite mashed potato recipe, although really just about any kind will do! The vinegar provides a distinctive tang, and sometimes I even add it to potatoes without doing the onions. This is adapted from a cookbook I've had for years 50 Best Mashed Potatoes. (No, I haven't tried all 50 recipes.)

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!)