Trim any excess fat or sinew from the lamb shanks, season with salt & pepper and seal in hot oil in a thick double bottom pot. Remove the meat and fry the vegetables in the same fat until slightly browned. Stir in the spices and tomato paste and cook for a couple of minutes. Return the shanks to the pot, stir in the tomato polpa, the herbs and lemon zest, cover with enough chicken stock. Cook the lamb in the sauce on gentle heat for approx. 1 hour. To prepare the lentil curry, cook the onion, carrot, garlic, ginger & chilli in hot oil for a couple of minutes. Stir in the curry, toast for a minute or two. Add the lentils and potatoes mix to coat with the spices, stir in the tomato polpa and cover with enough stock. Simmer until potatoes are soft and lentils are cooked, stir in the spinach before serving. Serve the lamb shanks accompanied with the lentil curry, topped with yogurt & lime wedges.