Preheat your slow cooker to high.
Heat the butter and oil in a large pan, add in the onion and chicken, and cook for 5-6 minutes until the onions are softened and the chicken is sealed.
Add in the garlic, ginger, salt, garam masala, curry powder, paprika and cinnamon. Stir and cook for 1-2 minutes.
Add in the stock, tinned tomatoes, tomato puree, sugar and cardamom. Stir then pour into your slow cooker and cook for 4-5 hours on high or 6-8 hours on low.
Remove the cardamom then stir in the cream and allow to heat through.
Serve over rice sprinkled with some coriander.
