Preheat the slow cooker by setting to Auto or Low.
Heat the oil in a frying pan and add the onion and garlic. Stir for 3 to 4 minutes until soft then add the chopped ginger and red chilli, stirring constantly. Add the curry paste to the frying pan and stir well to mix. Add the lamb and coat thoroughly, continue to cook for 5 to 8 minutes until the lamb is still a bit pink in the the middle. Add half of the stock and the chopped tomatoes and bring to a gentle simmer. Turn out the frying pan contents into the slow cooker then add the remainder of the stock, add a pinch of saffron, half the yoghurt and season.
Cover and cook on auto or low setting for 7 hours or on high for 3 to 4 hours.
When ready to serve, stir in the remainder of the yoghurt and some fresh coriander. Sprinkle some more coriander on top and serve with rice.