Preheat oven to 400 degrees F.
Place potato halves on baking sheet. Bake potatoes in middle of oven for about 1 hour or until tender when pierced with fork.
Meanwhile, in a saute pan melt butter and saute onions with ginger until onions are translucent. Set aside. Remove potatoes from oven and scoop out flesh into a food processor, pureeing in batches if necessary. Add onions and butter, and puree until smooth. Season with salt and pepper.
Recipe courtesy of Sara Moulton