Cut thin slices from the fillet, removing all fat and skin. Lard with larding fat, put them in a saucepan in hot butter, brown evenly on both sides, cover the saucepan and simmer very gently for about 10 minutes. Then add 1 gill of sour cream, and simmer for another 15 minutes. Remove the fillets from the sauce, and put on a hot dish. Stir 1 tablespoon of flour in the cream, add 1 tablespoon of chopped capers, 2 tablespoons of tomato purée, 1 or 2 tablespoons of stock, mix well, and pour the sauce over the fillets.