1. Remove the fat from the kidneys, only leaving a very thin sheet. Take off the nervous white part between the lobes. Blanch in boiling water.
2. Melt the butter in a casserole and fry the kidneys over a low heat during some moments. Add salt and pepper and put them into the oven (180 ºC) during 15 minutes.
3. Get the kidneys away from the oven. Cut them into small pieces and put them in a platter. Keep hot.
4. Add the wine and let it evaporate over a low flame. Add the mustard, the cream and the crumbled piece of stock cube. Boil during some minutes; add salt and pepper if necessary. Cover the livers with the sauce.