Leave the potatoes whole if they are small, or cut them into halves or quarters if they are large. Place them in a small saucepan with enough salted water to cover them by several inches. Bring to a simmer, then cover partially and adjust the heat to maintain a gentle simmer. Cook until a knife slips in easily, about 30 minutes. Drain, then return the potatoes to the warm pot.
While the potatoes cook, melt the butter in a small saucepan over medium heat. Add the green garlic and saute until softened, 3 to 5 minutes. Add the milk and cream. Season with salt and pepper to taste.
Mash the boiled potatoes. Add the hot milk mixture and stir until smooth.