I'm using small tuna for this, but a meaty white fish cut into steaks is a more common variation. Whatever fish you find at the local market that's fresh and meaty can be used, as long as it doesn't have a strong flavor of its own. The fish soaks up the curry flavors and added bamboo or coconut root soaks up the juice and provides much needed bulk to the curry.
The flavor you are aiming for is sour and spicey. Sour comes from the tamarind juice (tamarind paste mashed up in water so the water becomes sour, use the water in the dish to flavor the curry). The spicy comes from the curry paste that's so common in southern cooking. Be sure to taste and adjust the flavors at the end to get a good burn on your lips!
I threw in the fish head too for extra flavor, and some nice head meat. You don't need to do this, but why waste it?