Cleanse the lamb shoulder of fat and tendons and cut into large cubes. Marinate with the chopped ginger, garlic, thyme and cumin for 30 minutes.
Heat the oil in a large saucepan and fry the pieces of lamb and the onions cut into wedges. Add the peeled and pureed tomatoes, cinnamon stick, cloves, cardamom, saffron threads and chopped chilli pepper and let simmer for an hour. Add the peeled and quartered potatoes, the finely chopped mint leaves and the chervil and simmer for another 30 minutes.