For the filling, clean 1 stick of leek, cut in half lengthwise and wash. Blanch in boiling water for 1 minute, then drain. Wash the turkey meat and pat dry.
Blanched leek strips and cheese slices on top. Sprinkle with salt, pepper and grated lemon. Roll up the meat, pin it with wooden skewers and wrap it tightly with kitchen string. Place the meat on the fat pan.
Pour over hot clarified butter and bake in a preheated oven (electric: 200 °C/ gas: level 3) for approx. 1 1/4 hours. Peel onions, quarter them and spread around the roast. Meanwhile, for the vegetables, peel carrots and cut them into sticks.
Clean, wash and cut the rest of the leek into strips. Cook the carrots for about 5 minutes shortly before serving and the leeks for 3 minutes in salt water. Pour 400 ml water over the roast for the last 1/4 hour.
Wrap the roast in aluminium foil and let it rest for a short time. For the sauce, pour the roast stock through a sieve into a small pot. Add cream, bring to the boil. Season to taste with salt, pepper, mustard and lemon juice.
Swivel the vegetables in the hot fat and arrange them with the roast and some sauce on a preheated plate. Serve garnished with lemon wheels. Boiled potatoes taste good with it.