Put ingredients (A) into a food processor, blend to a fine paste.
Cook ingredients (B) in a nonstick saucepan for 15-20 minutes or until the flavour of the prawn and fish is extracted. Remove a little of the fish meat (if any) and add to the blended paste. Process again for 1 minute.
Heat 3-4 tablespoons oil in a nonstick pan, transfer the processed paste to the pan. Fry until fragrant. Add the thick broth from the cooked prawn shells and fish bones. Simmer until sauce is quite dry. Adjust with seasoning to taste. Dish out and put aside to cool in a clean jar for use later.