In a large bowl, mix together all the ingredients for the paste.
Prepare the lamb filet by removing any sinew and cutting into 1-1.5 inch (3-4cm) chunks. Add the lamb to the paste and stir to coat. Cover the bowl and set aside in the refrigerator to marinate for at least 30 minutes, or ideally overnight.
Heat a large skillet over a medium/high heat and sear the marinated lamb, along with the onions, until browned, then transfer to the slow cooker.
In another bowl, mix together the yogurt, coriander, sugar and chopped chili and then gradually stir in the tomato sauce and water.
Pour the sauce over the lamb and onions, then add the bay leaves, cardamom pods and cinnamon stick and ensure they are submerged in the sauce. Cover with a lid and cook on low for 6 hours or high for 3½ hours. When ready to serve, remove the bay leaves, cardamom pods and cinnamon stick
