Fish en papillote is the elegant-sounding name of a staple recipe of classic French cuisine. Translated into English, it becomes the much less elegant-sounding - "fish in a bag." By any name, however, this method of baking fish is a smash.
Typically, the fish is combined with vegetables and herbs, some butter or oil, and often some wine. All of this is wrapped up in a piece of kitchen parchment and baked. The parchment keeps the flavor and moisture trapped inside during cooking, allowing the juices from the fish and the other ingredients to mingle and become a wonderful sauce.
And because the parchment is stick-resistant, the recipe requires very little fat. The small amount of oil in this recipe is there for taste and texture only.
In this recipe, the relatively few ingredients I've added to the salmon are in the service of the sauce. But let's say you wanted to make a whole meal in a bag, sort of like a high-toned TV dinner. In that case, you could add some substantial vegetables, for example sauteed mushrooms, steamed cooked potato cubes, blanched broccoli or carrots.
If you do add vegetables, they'll need to be pre-cooked. The denser vegetables - such as carrots and broccoli - simply won't have time to get tender during the 10 to 12 minutes of cooking needed by the salmon. Similarly, if wetter veggies - such as mushrooms and spinach - aren't pre-cooked, they'll release too much liquid in the packet and water down the sauce.
The only tricky part about cooking en papillote is that you can't see when the fish is done. If you slice open the bag, you risk losing some of the delicious sauce that's coming together. My solution is to start with the basic rule of baking fish: In a 400 F oven, give it 10 minutes of cooking time for every inch of thickness.
When I'm ready to test whether the fish is done, I stick a very sharp thin knife right through the parchment and down through the fish. No or little resistance? The fish is done. Significant resistance? Bake it for a few more minutes.