Heat the vegetable oil in a large non-stick saucepan. Add the mustard seeds and, once they are popping, turn the heat down and add the onion and cinnamon. Cook the onion until golden (this takes around 8 minutes).
Meanwhile, using a blender, make a fine paste of the ginger, garlic and chillies with 50ml water. Add this to the cooked onions along with the powdered spices. Cook until reduced, then fry over a low heat for 2 minutes or until the oil is released.
Add the tomatoes and another 100ml water; cook until completely reduced and fry the paste for 4-5 minutes or until the oil comes out on top.
Stir in the coconut milk and 150ml water, bring to a gentle simmer and allow the flavours to marry and develop for 5 minutes.
Add the fish (or chicken) and cook until done, around 4-5 minutes. Taste and adjust the seasoning.
Serve with fluffy white rice from your Sakura or Zojirushi rice cooker and a chapati or naan to mop up the delicious sauce.