Raise the heat and add the ground cumin. Cook for a minute or two.
Add lime juice, coconut and enough water to bring the depth of the sauce to nearly a centimetre. Stir till coconut "dissolves" and mixture is fairly smooth. Once it starts bubbling reduce heat to very low and just stir occasionally.
Five minutes before you want to serve meal, add fish and increase heat till it just starts to simmer. For thin fillets, cook gently like this for two minutes then turn over and cook/poach for two minutes on the other side. Then turn heat off and leave for a minute while you get other stuff ready.
Serve by putting fish on plates and then sauce on top.