The answer to this question is 'it depends'. First of all, I wouldn't freeze the cooked dish, but depending on whether you are using fresh or frozen fish fillets and prawns you may be able to freeze the assembled dish. If you are making this dish with fillets of fish and prawns that have been previously frozen, then the answer is 'No, you can't freeze this dish before cooking it'. If you are making it with fresh (previously unfrozen) fish, then the answer is 'Yes, you can freeze it'. To freeze, assemble the dish and allow it to cool, then cover the dish with a layer of tinfoil and wrap it securely with plastic wrap, or place it inside a large plastic bag. Cook from frozen, as per the recipe instructions, but allow an additional 7 minutes of cooking time.