Step by step
Add the mullet to the sauce and cook without moving (so as to avoid it breaking up), until done.
Tips
Mullet should be cleaned, and above all de-scaled, very thoroughly: you should get your trusted fishmonger to do this because, being very delicate fish, they can easily fall apart.
Trivia
Mullet is not a widely available fish and it is therefore hard to find a good specimen at a low price.