Peel and slice the onion and cut the peppers into thick slices. Add these to the slow cooker along with the garlic, ginger and fresh chilli pepper. Remove any sinew from the chicken thighs, cut each into thirds and add these to the pot.
Spoon in the tomato puree, coriander, all of the spices (garam masala, paprika, cumin, chilli powder, cinnamon), salt, pepper, sugar and cornflour. Mix well to coat the chicken and vegetables in the tomato, seasoning and flour.
Crumble in the chicken stock cube then add the tomato passata and lemon juice, and mix. Add the bay leaves, submerging them in the sauce, taking care not to break them into pieces. Cook on low for 5-7 hours.
At the end of cooking time, stir the curry through in the slow cooker whilst pouring in the double cream. The sauce will blend to a lovely terracotta colour.
Serve with basmati rice and fresh coriander.
Making the sauce. When all of the ingredients are in the slow cooker ready to cook, the chicken and vegetables won't be submerged in the sauce - this is fine! As the curry cooks the chicken and vegetables release juices and create more liquid. Heat. If the curry is too hot for your palate due to the chillis you can add a little more lemon juice or cream to the sauce, alternatively serve with natural yogurt.Cooking time. There is some variance in the heat emitted by different brands and sizes of slow cookers which will affect the cooking time. The time listed on the recipe is a guide.Sauce. Due to higher degrees of heat in some slow cookers, the cooking sauce may reduce faster. To compensate for this a little more passata and cream can be added if required.