Al cartoccio in Italian is the culinary term for cooked in a paper parcel or a paper bag and versions of this dish are cooked all over Italy. You may also have eaten pasta al catoccio and in fact, recently in a restaurant in Melbourne, I ate a version of pasta cooked in a bag and it was made with braised mushroom.
Last time I was in Sicily I ate Fish in a bag in a restaurant in Sciacca (south west coast). They had added a little seawater and a few prawns and mussels to their version (see photo below); I have used black olives instead.
Traditionally the fish was wrapped in strong parchment-like paper, but metal foil has reached Italy and Sicily and has replaced the paper. I like to use foil, but I line the package with some baking paper – this prevents sticking and provides greater insulation. Adding seawater has been a common practice when cooking fish dishes and the restaurant called their version pesce in acqua di mare (fish cooked in seawater). If you live close to the sea, 1 tablespoon of seawater is sufficient.
Every time I have eaten this dish in a restaurant in Italy, the parcel was presented at the table and pierced by the waiter; he then separated the fish into fillets and served it on my plate.
I have used a whole Yellowtail Kingfish for this recipe and it was sufficient for two people.