Wash the meat, dab dry and season with salt and paprika. Roll up meat and tie up with kitchen string. Heat oil in a roasting pan. Fry the meat thoroughly while turning. Coat the fat pan of the oven with oil, place the meat on top.
Cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 2 hours. Peel onions and garlic and dice finely. Heat the roasting pan again, fry the diced onions and garlic for 1-2 minutes.
Add the tomato paste and roast for about 1 minute. Deglaze with red wine, cider and stock while stirring, reduce by about 1/4 at medium heat. Clean and wash the Brussels sprouts and cook in boiling salted water for about 15 minutes.
Pour sauce through a sieve into a pot and bring to the boil. Stir the starch and a little water until smooth and thicken the simmering stock. Season to taste with some salt, pepper and sugar. Put the spaetzle into a sieve, pour hot water over them (otherwise the spaetzle stick together very much), let them drain.
Melt 25 g butter in a pan. Fry the spaetzle for 3-4 minutes, add the hazelnuts, continue to fry briefly, season with salt and a little nutmeg. Drain the sprouts. Add 25 g butter, melt, toss and season to taste with salt and pepper.
Re-heat the sauce. Remove meat from the oven, cut into slices and serve with spaetzle, sauce and Brussels sprouts.