Ingredients
2 slices of belacan (unroasted) (4cm square, about 1.5cm thick)
450g tamarind pulp
550g Bokok fish or kembung fish, cleaned and gutted
1 bundle lemongrass (about 3-4 stalks, use the inner white bulbs only) (smashed)
3 1/2 tsp turmeric powder
Salt to taste
Chilli powder or red chillies to taste (optional)

Method
1) Break up the belacan and put into a large soup pot, together with two cups of water.
2) In a bowl, add one cup of water to the tamarind pulp. Squeeze tamarind and pour the juice into the soup pot. Repeating seven to eight times until the "juice" coming out is clear. Discard tamarind pulp.
3) Add fish.
4) Add lemongrass and turmeric powder. Add the chili powder or red chilies here, if desired. Stir.
5) Bring to a boil, then leave to simmer for at least an hour.
6) Add salt and chilli powder to taste.
Serve hot.

A word of caution: do not serve the soup in plastic bowls. The turmeric in the soup will leave a stain.

Thanks to Tummywise