Shred potatoes using a medium shredder, to make 3 cups. In a heavy skillet, melt butter over medium heat. Add onion and cook until transparent, about 5 minutes. Add potatoes and stir to blend; press down firmly. Cook over low heat for 25 to 30 minutes, until potatoes are crisp and brown on the bottom. Cut through potatoes from the edge of the pan to center. Carefully run a spatula under cut sections, flipping over one section at a time. Cook until browned. Run spatula under potatoes, loosening them from the skillet. Turn out onto platter. Sprinkle with salt and pepper.
Hashed Brown Potatoes serves 4.