Cut the loin into cutlets or the leg into slices. Melt some mutton fat in a saucepan, add the chopped onion and, when it begins to brown, put in the pieces of meat. Pour in some stock, season with salt and pepper, cover the saucepan and cook till the meat is half done. Next add the rice, previously scalded, and then pour cold water over it. Add more stock, peppercorns, a bayleaf, cover and cook till the rice is quite tender. It should be soft, but each grain should be separate.