Clean the mushrooms and cut them in half. Clean the leek, cut into rings and wash thoroughly. Wash the meat, dab dry and season with salt. Heat oil in a roasting pan. Brown the meat thoroughly all around. Add mushrooms and leek and fry briefly. Deglaze with cream, bring to the boil and braise for about 15 minutes.
Cool the roaster with its contents and put it in a cool place overnight.
Braise in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-30 minutes. Remove the meat, leave to rest briefly in a warm place and cut into slices. Serve with the creamed vegetables. Serve with rice.