In a frying pan, heat the oil and fry onions, garlic, and ginger until golden. This can take up to ten minutes.
Meanwhile, mix the cayenne, coriander, dry mustard, and turmeric in custard cup and set aside. Mix the water, tomato paste, and salt together and set aside.
Skin and wash the fish. Place in 1.5-liter flat dish (with lid) and note weight of fish.
Add the spice mixture to the cooked onions and stir around in the oil to heat. Add liquid and simmer till the gravy is well blended.
Pour over the fish in dish. Cover and cook in an 1100-watt microwave on HIGH (100% power) for 4 to 5 1/2 minutes per pound, until an internal temperature of 145 degrees F.
Let rest 5 minutes before serving.
Harvey Day, The Complete Book of Curries, 1970
Kiwi Fruit Chutney is a nice accompaniment.http://macheads2.net/recipes/KhariMachhli.html