Season the lamb with salt and pepper then brown the lamb in a frying pan for 5 - 8 minutes with a little olive oil. Remove the lamb and place in the slow cooker. Add the chopped onions to the frying pan and fry for around 10 minutes. Then stir in the garlic, spices, chillies and tomato puree and fry together for a further 1 minute. Then remove from the frying pan and add to the slow cooker.