Saute onions in butter until translucent. Set aside. Peel potatoes and cut into 1-inch pieces. Place potatoes in saucepan with cold salted water to cover and boil, covered, until very tender, about 15 minutes. Drain in colander. In a food processor, puree potatoes, milk, butter and Sauteed onions until smooth. Season with salt and pepper. Sprinkle with chopped candied ginger.;