In a custard cup, combine the cayenne, chilli, coriander, mustard, and turmeric; set aside. Chop the onion.
Heat oil in frying pan and cook the chopped onion and crushed garlic until the onion is golden. This can take up to ten minutes.
Stir in the spice mixture and heat until it gives off an aroma. Mix coconut milk with flour and stir in. Add the lemon juice and salt. Cook over low heat until the mixture thickens.
Meanwhile rinse the fish fillets and arrange in a 1.5-liter flat dish (with lid). Note weight of fish fillets.
Pour coconut sauce over fish in dish. Cover and cook in an 1100-watt microwave oven on HIGH (100% power) for 4 to 5 1/2 minutes per pound.
Let rest 5 minutes before serving.
Harvey Day, The Complete Book of Curries, 1970