Prepare the marinade by combining the onion, green pepper, tomato, coriander, thyme, ginger, garlic, turmeric, cumin, black pepper, tandoori masala, garam masala, chicken stock cube, yogurt, lemon juice, coconut milk, cooking oil, water, salt, and chili powder (if using).
Add the chicken thighs to the marinade and mix well to coat the chicken.
Cover and marinate for at least 30 minutes, or up to 24 hours in the refrigerator.
Cook the chicken in the marinade, either on the stovetop or in the oven, until the chicken is cooked through and the sauce has thickened.
Serve the chicken curry with cooked basmati rice.