Heat some oil in a large pot and get it quite hot. Add the prawn heads and fry, squashing them down to squeeze out the insides. Keep frying till golden brown in colour. Add the quartered onion and cook for another 5 minutes.
Add the rest of your fish heads and carcasses.
Add the wine and stir the bottom to make sure you have removed all the flavour bits. Bring to the boil then top with water to cover the seafood and place in the bay leaf. Bring to a mild simmer and cook for 40 minutes, skimming off impurities as it cooks.
Strain through a fine sieve and now it's ready to be used in your preferred recipe.
Recipe by Finding Feasts at https://www.findingfeasts.com.au/recipe-index/fish-stock/