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) -