Wash and peel potatoes and cut into paper thin slices. Pat dry with towel. Cut onion into paper thin slices. Heat half of the bacon fat in a heavy frying pan and fill the pan with alternate layers of potatoes, onions, and cheese, finishing with potatoes. Sprinkle each layer with salt and pepper. Dot the final layer with remaining bacon fat. Cook over moderate heat until potatoes are almost tender. Turn the Haggerty carefully onto a plate and then carefully slide it back into the pan and continue cooking undil done. To serve, cut into wedges and serve with a dollop of sour cream..