If you are using the starter, which will give a thicker, smoother soup, early in the morning of the first day, put 2teaspoons each of the rye and bread flour in a mixing bowl and mix together. Pour over atablespoon lukewarm water and mix well. Cover the starter with paper towels and set aside in a warm place (22–25°C/72–77°F).
During the morning of the following day, “feed” the starter with the remaining flours and about 2teaspoons lukewarm water, stirring very well to combine. Set aside, covered as above, for a further 8 hours.
Rub the lamb shank with the cardamom, cumin, smoked paprika, ground coriander and cinnamon and season with some salt. Set aside.
With a sharp knife, cut a cross in the skin of each tomato, then put them in a heatproof bowl and cover with boiling water. Leave to stand for 2–3 minutes or until the skins have split, then drain. Plunge into cold water to stop them cooking, then peel off the skins and discard. Slice in half and scoop out the seeds, then finely chop the flesh.
Melt the butter in a heavy-based saucepan over a medium heat. Add the onion, cover the pan and reduce the heat to low, then leave to sweat, stirring often, for 5 minutes or until softened.
Increase the heat to medium, add the lamb and any loose spices and sear for 3 minutes on each side. Add the garlic and ginger and cook for a further minute until aromatic, then add the tomatoes, stock, saffron, if using, chickpeas, lentils and bay leaf.
Cover the pan, increase the heat to high and bring to the boil, then reduce the heat to medium-low and simmer, covered, for 1 hour or until the lentils are soft and the meat is tender. Discard the bay leaf.
Remove the lamb from the pan and cut away the meat into small bite-sized pieces, then return the meat to the pan with the bone. You can extract the marrow with a narrow spoon or skewer, if you like.
Dilute the starter, if using, with 100ml/3½fl oz/scant ½cup water, stir well, then slowly pour it into the pan, stirring for about 20 minutes or until the mixture has thickened. Stir in the preserved lemon and coriander/cilantro and season with pepper. Ladle into bowls, sprinkle with coriander/cilantro and serve with dates, lemon quarters and warm Arabic Bread, if you like.