Heat the oil in a large saucepan, add the lamb and fry until browned on one side, turn it over and add the onion. Cook until the lamb is browned all over and the onion just beginning to colour.
Stir in the garlic, spice blend and ginger, then the stock, lentils and salt and pepper. Bring to the boil then reduce the heat, cover and simmer for 1½ hours.
Add the sweet potato and carrot, bring back to a simmer then re-cover and cook for 1 hour. Lift the lamb out of the soup with a draining spoon, put on to a plate then carefully remove bones and excess fat, breaking the meat into small pieces. Return the meat to the pan and reheat if needed. Taste and adjust the seasoning if needed.
Ladle the soup into bowls, sprinkle with torn coriander leaves and serve with hot fennel flat breads.