Wash the meat, dab dry and season all around with salt and pepper. Place on the fat pan of the oven and bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for approx. 1 hour.
In the meantime, clean, peel, wash and cut the soup greens into pieces. Peel and wash the potatoes and also cut them into pieces. After half of the frying time, place them around the roast on the fat pan.
Deglaze gradually with 1/4 litre of stock and let it braise. Sprinkle the bartens with coloured pepper about 10 minutes before the end of the cooking time. In the meantime, put the remaining stock into a pot and bring it to the boil.
Add sour cream and horseradish and stir in. Mix starch and 1-2 tablespoons of water until smooth, stir into the sauce and bring to the boil. Season to taste with salt and pepper. Wash parsley, dab dry and cut into fine strips, except for a little bit for garnishing.
Let the roast rest for a while, then cut it open and arrange it on a plate with the vegetables. Sprinkle with parsley strips. Serve garnished with remaining parsley leaves.