It’s really worth making fish stock if you have either prawn shells or the remains of a whole fish but you need to make it as soon as possible as the shells and the fish bones etc. deteriorate very quickly. You could store them, well covered, in the fridge for one day but no longer. No specifics about quantities given here but you will need the shells, including the heads and legs from at least 8 large uncooked prawns or the bones, skin and head of a fairly large fish with an original weight of around 1.5kg. It is a bit messy but hopefully you will think it worth the effort.
Put the prawn shells, heads and legs or the remains of the fish, skin, bones and head, in a pan and cover with cold water. Set the pan over high heat and bring the whole lot to the boil. Reduce the heat so that the water is bubbling very gently for 15 minutes only.
Remove the pan from the heat and drain the stock through a colander into a bowl – wrap the shells or bones etc. and discard them – this is the messy bit.
You need to use the stock within a day or two of making it, or you can freeze it, but use it within three months. To save freezer room, reduce the stock by rapid boiling in a wide pan which will also intensify the flavour. Let it cool and then freeze the stock in small containers (ice cube trays are good) so that you can use a little at a time.
Use the stock in fishy pasta sauces, or risottos or soups or stews.