Put the potatoes and garlic in a pot, cover with cold water and bring to a boil. Cook over moderate heat until the potatoes are tender, about 20 minutes. Drain the potatoes and garlic and return them to the pot. Shake the pot over moderate heat to evaporate any water. Press the potatoes and garlic through a ricer into a large bowl and stir in 12 tablespoons of the butter. Season with salt.
Preheat the oven to 425°. Transfer the potatoes to a 3-quart baking dish and use the back of a spoon to make indentations in the surface. Melt the remaining 2 tablespoons of butter and brush all over the surface of the potatoes.
Bake the potatoes for about 30 minutes, until golden on top.
Meanwhile, in a small saucepan, whisk the crème fraîche with the horseradish. Season with salt and white pepper and warm over moderately low heat. Transfer to a small serving bowl.
Serve the mashed potatoes, passing the horseradish cream on the side.
The recipe can be prepared through Step 2, covered and refrigerated overnight. Bring the potatoes to room temperature before baking.