A few things to keep in mind, you can use foil if you don’t have parchment, but be careful with acids and wine they can leach strange flavors out of the foil. Fish is the most common thing to be cooked en Papillote but it’s far from the only thing you can make this way, check out this cool article
from Amanda Hesser at NY Times on other options for the parchment envelope. The best thing about this method is the fact that you whole house will not smell like fish, just throw the empty parchment away and the smell goes with it. It’s your choice you can serve the fish to each guest in the envelope or slide it out on the plate, just be sure you don’t lose any of the wonderful broth you have created in the package from the wine, capers, and butter if you plate the fish.