Combine all of the marinade ingredients in a mixing bowl and stir thoroughly.
Roughly chop the chicken thighs into large chunks, and add to the marinade mixture, coating the meat thoroughly. Seal and refrigerate for a minimum of 30 minutes (2-3 hours ideally).
Cook chicken and sauce
Heat the oil in a pan over a high heat, then briefly fry the marinaded chicken pieces. This will create a brown coating on the chicken as the marinade cooks, recreating the charred effect usually achieved in a tandoor. Add the chicken to the slow cooker, along with any marinade left in the bowl.
Roughly chop the onion and pepper, crush the cloves of garlic, and add to the slow cooker.
In a separate bowl combine the ingredients for the sauce, mixing well. Pour the sauce over the meat and vegetables in the slow cooker, ensuring everything is coated in the mixture. Cook for 4 hours on a high setting, or 6 hours on low, stir and then serve with rice and a sprinkle of freshly chopped coriander.