Prep the components:
Chop the tarragon with the butter. This way, the leaves are immediately coated and don’t turn black. If you’re using parsley, this isn’t an issue. Just chop the parsley fine and work it into the butter in a bowl with a wooden spoon.
Sauté the mushrooms over high heat in the olive oil until they turn brown and smell fragrant, usually about 10 minutes. If they release liquid, cook them until the liquid evaporates. A minute before you take them off the heat, season them with salt and pepper and sprinkle with the chopped garlic. Let cool completely.
Assemble the papillotes:
Fold the paper in half as you would a book and press along the fold to make a crease. Open the paper and set a piece of the fish in the center of one half of the paper. Top with a quarter of the mushrooms and herb butter and sprinkle with 2 Tbs. of the wine.
Brush a 1-inch border around the filled half of the paper with some egg white lightly beaten with a little salt (the salt helps to thin the white) and fold over the other half of the paper. Press on the edges of the paper so the egg white seals it. (If you’re using aluminum foil, skip the egg white but fold in the same way.)
Brush the edges of the paper a second time with more egg white and make a series of little folds all around the edge to create a seal. Brush one more time with the egg white and make a second series of folds to make the seal airtight. Repeat with the remaining ingredients to make three more papillotes.
Bake and serve:
Arrange the papillotes on a baking sheet without overlapping. Bake in a 425ºF oven for about 10 min. per inch of thickness of the fillets. Serve immediately, as they’ll deflate as they cool.
nutrition information (per serving):
Photo: Scott Phillips