Heat the butter in a deep saucepan or medium sized wok. Add the onions, garlic, chillies, turmeric, fenugreek, cardamons and cinnamon and fry for 5 minutes.
Add the salt, cumin, ground coriander and garam masala and stir for a few seconds. Add the tomatoes and cook until it begins to thicken, then add the chicken and cook for 3 minutes before adding 250ml of just boiled water.
Simmer for 6 minutes until the sauce has reduced and is reddish brown in colour. Stir in the root ginger and sprinkle over the coriander.