To make the sambal bajak, place the tamarind paste in a heatproof bowl and pour over 150ml boiling water. Allow to cool, then combine well. Strain the mixture through a fine strainer, extracting as much liquid as possible. Discard the solids. Place the chillies, spring onions, galangal, ginger, lime leaves, garlic, lime juice and salt in a mortar and pestle, and grind to a coarse paste. Add the sugar and terasi paste and grind until well combined.
Heat the palm oil in a wok over medium heat. When hot, add the paste, and stir-fry for 2-3 minutes or until fragrant. Stir in the tamarind liquid and simmer until reduced by half. Remove from heat and cool.
Using a sharp knife, score the fish on both sides, making sure the incisions do not go all the way through to the bone.
Heat a char-grill or wood barbecue to high, then make sure the coals have burned down so there is no flame before cooking the fish. Brush the fish on both sides with the coconut and chilli oil, then place on the grill and cook for about 6 minutes on each side, basting the fish each time you turn it. (The cooking time will vary depending on the thickness of the fish. To check whether the fish is cooked, press it lightly just below the head. If it gives, then the fish is done.)
Serve the fish on a large platter scattered with thinly sliced shallots, chopped chilli, bean sprouts and lime wedges with the sambal passed separately.