Cooking en papillote simply means that you are cooking something “in parchment” (or in foil or in banana or bamboo leaves). By wrapping portions of protein alongside herbs, vegetables, and some sort of liquid in a secure parchment bundle, the heat of the oven will cook the protein and infuse it with the flavors of its adornments. The addition of a liquid is necessary in the wrapped parcels as the oven’s heat will create a steam that prevents the protein from drying out while also imparting considerable flavor. We also suggest adding a bit of fat to your bundles—in the form of cream, butter, or olive oil—because, as we all know by now, fat equals flavor.
These guidelines appear general because they are. The particulars are really up to you and your personal preferences or what is available to you on the day you are cooking. All you really need to remember is that thicker cuts of protein (swordfish or chicken, for example) will call for a longer time in the oven and the vegetables you add to your parcels ought to require the same cooking time, so choose and chop accordingly. The most fun aspect of cooking this way, besides the lack of cleanup involved, is that en papillote might as well mean “anything goes” (it doesn’t) because this is truly a “choose-your-own-adventure” cooking experience. And, it’s a healthy way to cook too. Yes, even if you add un peu de beurre.