For the filling, boil a good 200 ml of salted water. Sprinkle with couscous. Remove the pot from the heat and let the couscous swell in the closed pot for about 10 minutes. Clean, wash and cut leek into rings.
Peel and chop the garlic. Heat the oil. Sauté leek and garlic for about 5 minutes. Wash parsley, shake dry and chop leaves finely. Wash the lemon hot, dry it and
Squeeze the lemon. Loosen the couscous with a fork. Add egg, parsley, lemon peel and lemon juice. Fold in leek and garlic. Season to taste with salt and pepper.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Wash meat, pat dry and process further. Add the couscous filling and stuff firmly with the handle of a wooden spoon.
Season the meat all around with salt and pepper. Peel and roughly dice the onion. Put meat and onion in a roaster. Fry in a hot oven for about 1 hour. After approx. 30 minutes pour on 1⁄4 l hot water.
For the relish, peel, quarter, core and dice apples. Heat butter. Steam apples and sugar in it for about 5 minutes. Deglaze with cider and vinegar, bring to the boil and simmer open for about 10 minutes.
Season to taste with cinnamon.
Turn up the oven (electric cooker: 250 °C/circulating air: 225 °C/gas: see manufacturer). Mix 2 tbsp. salt and approx. 6 tbsp. water. Brush the crust of the roast with it and continue roasting for approx. 10 minutes until the crust is crispy and bubbles appear.
Brush once or twice with salt water. Cut the roast open and serve with the relish. Mashed potatoes taste good with it.