Wash the fish fillet and pat them dry. Marinade the fillet with salt and juice of 1/2 a lemon and keep aside for ten minutes.
In the meantime, prepare the sauce.
Take a frying pan and add coconut milk or cream. Keep the heat at medium to low.
Add freshly grated ginger, green chilies, lemon juice, fish sauce and sugar and let the sauce thicken a bit.
Then add the marinated fish and place them gently around the pan. If it is a thick slab, you will have to turn the fish around after a couple of minutes. It takes 3- 4 minutes for a thick slab to cook and 2 - 3 minutes for a thin slab.
After adding the fish, add grated lemon rind and chopped coriander. Stir around and the minute the fish is cooked, turn off the heat.
Serve poached fish with steamed rice. If you have a thinner consistency, you can have it as a soup too.