Leave skin on or peel potatoes. Cut into 1-inch chunks. Place potato pieces in medium pot and pour over enough water (or reduced-sodium broth) to cover.
Set pan over high heat and bring to a boil. Boil 10 minutes, or until tender. Drain, then shake potatoes over low heat 1 minute to dry. Mash with a potato masher or fork.
Mix in buttermilk, bacon and onions. Season with salt and pepper.