A fish cooked en papillote is wrapped in parchment paper, and baked in a medium oven so it gently steams in its own juices. Aromatics, such as dill, sliced onions, and olives, are also enclosed in the package to flavor the fish. You may substitute aluminum foil for the paper, but if you do so, avoid using acidic ingredients like lemons, tomatoes, and white wine, which will react with the foil and cause an unpleasant taste. When the fish is done, the parchment will puff dramatically (if you use foil, the fish will cook more quickly and the foil will not puff up, so check it early).
Baking Fish en Papillote (in Parchment)
Martha Stewart Living, May 1995