How to Make It
Place potatoes in a large Dutch oven with cold water to cover by 2 inches. Bring to a boil over medium-high; boil until potatoes are tender, about 25 minutes. Drain and return potatoes to Dutch oven over medium. Cook, stirring once, until potatoes dry out slightly, 30 to 50 seconds.
Place buttermilk and butter in a microwavable glass bowl, and microwave on HIGH until warm, 30 to 60 seconds. Add warm buttermilk mixture, crème fraîche, salt, and white pepper to potatoes, and mash with a potato masher to desired consistency. Keep potatoes warm.
Meanwhile, cook bacon in a medium skillet over medium-high until crisp. Drain bacon on paper towels, reserving drippings in skillet. Crumble bacon. Cook 1 cup diagonally sliced scallions in hot drippings in skillet until lightly browned and crispy, 3 to 4 minutes. Transfer scallions to paper towels with a slotted spoon to drain. Sprinkle scallions and crumbled bacon over warm mashed potatoes.
Spoon potatoes into a 4- to 6-qt. slow cooker. Cover and cook on HIGH until heated through, about 2 hours. Or reheat in a tightly covered baking dish at 350°F until heated through, about 1 hour.