4 (6-ounce) thickly cut filet mignons, trimmed New York Strip or Chateau Briand 1 teaspoon salt 1/2 teaspoon freshly ground black pepper 1 tablespoon olive oil (OPTIONAL 4 (1-ounce) slices goose or duck liver, or pork country-style pate) One-half (17 1/4-ounce package) frozen puff pastry (1 sheet), thawed Mushroom Duxelles, see recipe 1 large egg beaten with 2 teaspoons water to make an egg wash
Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper and set aside. Season both sides of each filet with 1/4 teaspoon of the salt and 1/8 teaspoon of the pepper. Heat the oil in a large heavy skillet over medium-high heat. Add the filets and sear for 1 minute on each side for medium-rare. Transfer to a plate to cool completely. Roll out the puff pastry on a lightly floured surface to a 14-inch square, and cut into 4 (7-inch) squares. Spread one-quarter of the mushroom duxelles on top of each filet and top each with 1 slice of the pate, pressing to flatten.
Place one filet, mushroom side down, in the center of a puff pastry square. Using a pastry brush or your finger, paint the inside edges of the pastry with egg wash. Fold the pastry over the filet as though wrapping a package and press the edges to seal. Place the packages seam-side down on the prepared baking sheet. Brush the egg wash over the tops and sides of each package and bake until the pastry is golden brown and an instant-read thermometer registers 140 degrees F for medium-rare, about 20 minutes. Remove from the oven and let rest for 10 minutes before serving.