PREHEAT THE OVEN to 400 °F.

Cut four sheets of foil and four sheets of parchment, each 8"×12".

Arrange the foil pieces seperately on your work surface; top each with a piece of parchment. (minute 0-2)

SEASON THE FILLETS on both sides with salt and white pepper; place each on a piece of parchment. (minute 2-3)

TOSS THE MUSHROOMS with garlic and thyme. Season with salt white pepper and add the oil; toss gently. Place the mushrooms on fillets, dividing them evenly amount all four. Top each with 1 teaspoon (tsp) butter. (minute 3-5)

PULL THE LONG SIDES of the paper and foil to the center and fold over to close. Fold the ends under to close each packet tightly. (minute 5-7)

ROAST THE PACKETS on a baking sheet until they begin to puff with steam, 12 to 15 minutes. Open, squeeze lemon juice over each fillet, and serve immediately. (minute 7-19)