Onions peel, halve and in thin strips cut. Wash the lamb, dab dry and cut into cubes. Heat the oil in a frying pan, preferably in the tajine. Fry the meat in it in portions all around for about 5 minutes, then add the onions in portions and fry for another 3 minutes.
Season with salt and pepper.
Add Ras-El-Hanout and tomato paste and sweat briefly. Add stock and simmer covered for about 1 1/4 hours. Peel, wash and dice the potatoes. Add potatoes to the meat halfway through the cooking time.
Wash the tomatoes, cut them in half. Add plums and tomatoes to the meat about 15 minutes before the end of the cooking time. Wash the coriander, shake dry and chop the upper 1/3 of the stems coarsely. Add half of the coriander to the meat, season again with salt and pepper to taste.
Sprinkle with remaining chopped coriander and serve.