Wash the turkey leg and dab dry. Cut the mozzarella into thin slices and place them on the meat, except for 3 slices. Place the slices of bacon on top. Wash sage leaves, dab dry, spread on the bacon and place remaining mozzarella slices on top.
Fold up the meat, pin the edges together with wooden sticks and wrap them with kitchen string. Rub with salt, pepper and oil. Place meat on the fat pan of the oven and roast in the preheated oven (electric: 200 °C/ gas: level 3) for approx. 1 3/4 hours.
Peel and quarter the onion and put it on the fat pan. Pour 1/2 litre of water over the whole roast at intervals. 15 minutes before the end of the cooking time, wash the tomatoes and cut them into slices. Clean, wash and cut Chinese cabbage into strips.
Melt the fat in a pan. Brown the hazelnut leaves in it at medium heat. Add the prepared vegetables and toss in for about 5 minutes. Season with salt and pepper. For the sauce, melt the remaining fat in a saucepan.
Sprinkle with flour, sweat briefly. Add chicken stock while stirring, bring to the boil. Add processed cheese and let it melt. Remove the roast from the oven and keep covered warm. Pour the roast through a sieve into the sauce.
Bring to the boil and season with salt and pepper. Cut the roast open and arrange on a plate with the vegetables. Serve sauce extra and sprinkle with coloured pepper. Garnish the roast with parsley and sage as desired.