Preheat the oven to 450 degrees F (230 C).
Trim the base of the stem of each mushroom and then scrape the stems to remove any dirt. Wipe the caps clean with a damp paper towel and quarter the mushrooms through the stem end.
Fold an 18-inch (45-cm) long sheet of parchment paper in half crosswise. Unfold the sheet and place the fir sprigs on the bottom, near the fold. Lay the mushroom quarters on top of the fir sprigs.
Rinse the foie gras under cool running water and pat dry with paper towels. Trim away any blood and green spots. Using a small sharp knife, score the foie gras on the larger 2 sides with crosshatching, cutting just barely into the liver, and then season with salt and pepper. Season the mushrooms with salt and pepper and place the foie gras on top of the mushrooms.
Fold over the parchment paper to cover the contents. Then, starting at one corner, turn the edges of the parchment over to make a small fold. Continue to make small, overlapping folds around the entire edge of the parchment package, sealing it securely closed.
Place the parchment package in a 12-inch (30-cm) ovenproof sauté pan, place the pan over medium heat, and cook until the parchment begins to puff, about 1 minute. Immediately place the pan in the oven and cook the package for 12 minutes. Listen to hear the sizzle inside the parchment.