Prepare a wrapping
Using aluminum foil, on the other hand, makes it much easier to create a tight seal. It's possible the acidic ingredients will react with the aluminum, but after short cooking periods, I didn’t have any problems.
Soaked bamboo and banana leaves also work well, not to mention look attractive.
Do not use wax paper or plain brown paper. They can leak and burn.
Choose lean, tender protein
Other options: lean proteins like whole stuffed fish, shellfish, chicken breasts, firm tofu, and pork chops. You might consider searing meats for color and flavor before wrapping them up.
Cuts of fish and meat should be no thicker than about an inch.
(Vegetables work too, but more on that later.)
Select flavorful accompaniments and keep it simple
Some options: scallions, fennel, ginger, red onion, shallots, mushrooms, herbs, olives, capers, orange slices, peppers, asparagus, fresh peas, garlic, lemon zest.
Be careful not to crowd the package with too many, especially those with a high water content. You could wind up with a soggy result.
Create a steamy environment with additional liquids
Drizzle in liquids that pair well with your other ingredients: reduced stock, coconut milk, wine, vinegar, soy sauce, citrus juice, or fish sauce.
Fats such as butter, olive oil, sesame oil, and cream will also add moisture and balance.
Fold it up tight to keep the moisture in
If you get a tight seal, the package will swell up like a balloon when it cooks.
Watch it puff up in the hot oven
Determining the right temperature and cooking time may take a little experimentation. Thinner fish fillets can cook in under 10 minutes at 425°F. A chicken breast will need about 20 minutes. Other factors like the amount of added vegetables and whether or not the fish is skinned will also affect the cooking time.
How can you tell when it's done? Look for a puffed-up, slightly browned parchment package. Open it carefully, take in the fragrant steam, and serve very hot.
En Papillote Veggies
Beets come out tender and easy to peel. Toss them with oil, salt, and pepper, wrap in foil, cook for about an hour (more or less, depending on the size) at 350°F.
En Papillote Limitations
Sliced onions, peppers, and a sprinkling of smoked paprika would’ve added flavor to the steam, but even then, I’d want them roasted. A covered casserole dish often does the trick. Like in the case of chicken breasts over (lightly pre-cooked) pasta in a tomato sauce.
To hell with it, just throw it all in one big pot. Mussels with a good amount of garlicky, wine-laced broth, for example. Having to sweat the aromatics first and being able to contain only so much liquid en papillote, makes it a bit of a hassle.
Still, I’d put mollusks with fish en papillote.