Wash the meat, dab dry. Heat oil in a roasting pan. First fry the leg on the skin side, then on the meat side for 5 minutes. Season with salt, pepper and paprika on all sides.
Pour apple juice. Braise in a closed roaster in a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: level 2) for approx. 1 1/2 hours. Peel, wash and cut carrots and parsley roots into thick slices.
Clean the leeks, cut them into pieces of about 1.5 cm and wash them. Peel celeriac and dice roughly. After 45 minutes, place the vegetables around the meat in the roaster and continue cooking without a lid. Pour on broth 15 minutes before the end of cooking time.
Meanwhile, prepare the spaetzle in boiling salted water according to the instructions on the packet, drain. Remove the meat, wrap in aluminium foil and leave to rest. Pour the vegetables into a sieve, catching the stock in a pot.
Stir the starch into 4 tablespoons of water until smooth. Boil up the stock and thicken with starch. Season to taste with salt and pepper. Put the vegetables back into the sauce. Melt the fat in a large pan. Swivel the spaetzle in the hot fat and season with salt and nutmeg.
Arrange the meat on a plate with vegetables and sauce. Spätzle extra rich.