Method

• Heat your slow cooker to High.
• Trim off all fat from chicken thighs and discard. Cut each thigh in half, or into thirds if large, bearing in mind they will shrink as they cook. Refrigerate until needed.
• Put onion, ginger, garlic and chilli into food processor and process until you have a chunky paste.
• Heat oil in large frying pan over medium heat. Add paste and cook for 8-10 minutes, until liquid from onion has evaporated and paste looks dry. Stir through butter and cook for 1 minute until butter has melted. Stir in chilli powder, cumin, coriander, garam masala, paprika and cardamom pods.
• Add 2 Tbsp cold water and stir to incorporate any stuck-on bits.
• Stir in tomato paste and stock powder, then scrape contents of frying pan into bowl of slow cooker. Add crushed tomatoes, cornflour, yoghurt and chicken. Give everything a good stir, nudging chicken into sauce, then cover and cook for 2 hours.
• Give a stir, then quickly cover again to avoid losing too much heat. Cook for further 2 hours, until chicken is very tender and sauce is thick.
• Serve butter chicken on cooked brown basmati rice, garnished with chopped coriander and with lemon wedges on side.