Slow cooker chicken tikka masala

Name: Slow cooker chicken tikka masala
Ingredients: 8-12 boneless, skinless chicken thighs each cut into 3 chunks; 2 tbsp veg or rapeseed oil; 1 large onion chopped; 2 garlic cloves crushed; thumb-sized piece ginger finely grated or chopped; 3 tbsp tikka curry paste; 500ml passata; 1 tbsp tomato purée; 1 tbsp malt vinegar; 1 tbsp light brown soft sugar; 1 cinnamon stick; 5 cardamom pods; 100ml double cream; handful chopped coriander; cooked basmati rice lime wedges and naan bread, to serve (optional)