Heat the oven to 140C (fan)
Toast the cumin and coriander seeds in a large heavy bottomed pan for a couple of minutes and then transfer to a spice grinder (or pestle and mortar) along with the black peppercorns and grind.
In the heavy bottomed add the vegetable oil and on a medium heat brown the lamb shanks on all sides and then remove them and set aside.
Add the spice paste ingredients to a small blender and blend until you have a smooth paste. Stir in the ground spices and then set aside.
Add the chopped onion to the pan and simmer for 5 minutes until golden and then add the spice paste, cinnamon, cardamon pods and cloves and cook for another 5 minutes.
Add the lamb shanks, 500ml of boiling water and half the tomatoes to the pan and bring to a simmer. Put a lid on the pan and then transfer to the oven and cook for 3 hours, turning the lamb shanks every hour or so. After 3 hours remove the lid, add the remaining tomato quarters, and cook uncovered for another hour (turning the shanks every 15 minutes) to reduce the sauce down.