For the filling, clean and wash the peppers. Peel the onions. Cut both into fine strips. Heat 2 tablespoons of oil in a frying pan. Steam onions and bell peppers for about 8 minutes. Season with salt and pepper and let cool off.
For roasting, dab meat dry and cut deeply into the meat on the upper side at a distance of 1-1 1⁄2 cm lengthwise, but do not cut through.
Rub half the kebab spice into the incisions. Then distribute the filling into the incisions. Tie the filled neck with kitchen twine into shape. First coat the roast with 2-3 tablespoons of oil, then rub with the remaining spice. Place the roast on a fat pan. First roast in the preheated oven (electric cooker: 250 °C/circulating air: 230 °C/gas: see manufacturer) for approx. 30 minutes. Then switch the temperature down (electric cooker: 160 °C/circulating air: 140 °C/gas: see manufacturer) and roast for approx. 2 1⁄4 hours.
Wash the potatoes and cook in water for about 20 minutes. Drain the potatoes, let them cool down a little, then peel them.
For the tzatziki, peel and finely chop the garlic. Wash and finely grate the cucumber. Mix yoghurt, cucumber and garlic. Season with salt and pepper.
For the potatoes, wash the rosemary, shake dry and pluck roughly into pieces. Heat clarified butter in a large pan. Fry the potatoes until crispy all around. Add the rosemary and fry briefly. Season with salt and pepper.
Take out the kebab roast and let it rest for about 5 minutes. Remove kitchen string and cut the roast into thin slices. Serve with rosemary potatoes and tzatsiki.
Homemade kebab spice: Finely crumble 2 brown cakes (e.g. "Kemm'sche Kuchen"). Mix with 3 teaspoons sweet paprika, 2 teaspoons salt (e.g. Fleur de Sel), 1 teaspoon coarsely ground black pepper, 1⁄2 teaspoon garlic powder, 1 teaspoon each of dried thyme and oregano and 1⁄2 teaspoon chili flakes.