Put the oil in a pan and add onion, garlic, ginger, chopped chillis and cumin. Fry gently until soft. Add the lemon rind and lemon juice. Stir around then add the coconut milk. Cook until hot. Add salt and pepper to taste. Add the fish and put a lid on the pan. Cook gently for 5-10 mins depends how thick your fish is. Check it is cooked by seeing if it flakes easily. Really important to get fish just cooked and not over cooked.