Put the soaked tongues in a large pan, covering them with some fresh water. Bring up to the boil, then reduce the heat and simmer gently, with the lid on, for about half an hour. Drain away the cooking water and let the tongues cool down enough so that you can remove the skin.
Heat the lard in a large saucepan or flameproof casserole. Throw in the onion and fry it for a couple of minute