Heat the butter in a frying pan over a low-medium heat and gently fry the onions for 10 minutes until soft but not coloured
Add the garlic, ginger, garam masala, curry powder and fry for a minute more, then add the puree to the onions and fry for another minute
Add all of the other ingredients except the yoghurt to the slow cooker
Cover with the lid and cook on medium for 4 hours
At the end of cooking, stir through the yoghurt and serve with naan or rice, if liked
