This is a special dish for the brief time when fresh ceps are available. It does not work with dried mushrooms, so don't be tempted to substitute them; If you cannot get fresh ceps then it is better to use large flat-cap field mushrooms.
You can use the same cooking method with brill, halibut and cod, though the last is more delicate and demands greater care. Note that when roasting fish you do not want the pieces touching or they will stew. Also, the heavier