Mix all marinade ingredients in a sealable container. Add chicken and marinate in fridge for at least 3 hours (ideally overnight)
Add small amount of boiling water to cashews, soak 5-10 minutes, then blend into a thick smooth paste
Melt ghee in large pan or pot with lid
Add cumin seeds and cardamom, fry until they make popping noises
Add onion and fry 8-10 minutes
Add ginger and garlic and cook until aromatic
Add green chilli and cook for a minute
Add cashew paste, stir until thickened
Add salt, chilli powder, cumin and coriander. Cook for 1 minute
Add tomato puree. Cover and simmer for 15 minutes
While sauce simmers, cook chicken for 7 minutes or so a side under a heated grill on alfoil
Add water to sauce if too thick. Remove cardamom and blend
Add chicken to sauce and simmer a further 10 minutes
Add finishing ingredients to taste