Wash the meat and dab dry. Spread with half of the fat and rub with salt and pepper. Clean, wash and chop the soup greens. Put meat and vegetables on the fat pan of the oven.
Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 1 hour. Add stock after 30 minutes. Mix crème fraîche and lemon juice and pour over the roast. Soak mushrooms in 1/8 litre lukewarm water.
In the meantime clean, wash, quarter and cut the carrots into pieces. Cook the carrots, broccoli and Brussels sprouts in a little salted water for 10-15 minutes. Remove the veal kidney roast and let it rest. Sieve the roast stock, add mushrooms with the liquid and bring to the boil, thicken with sauce thickener and bring to the boil again briefly.
Season to taste with salt and pepper. Drain the vegetables and toss in the remaining fat. Cut the roast into slices. Serve with vegetables and some sauce. Serve the rest of the sauce separately. Boiled potatoes taste good with it.