Preheat the oven to 400 degrees F. Line a large baking sheet with aluminum foil and grease the center with the butter.
Season the pork tenderloin on all sides with the Essence. Heat the oil in a skillet or Dutch oven, large enough to hold the tenderloin, over medium-high heat. Add the tenderloin and reduce the heat to medium. Sear evenly on all sides and cook, turning frequently, until the meat reaches an internal temperature of 110 degrees F, 16 to 18 minutes. Remove from the pan and cool for about 10 minutes.
On a lightly floured surface, roll out the pastry to a 12 by 18-inch rectangle. To make an egg wash, beat the egg with 2 teaspoons of water in a small bowl.
Evenly coat the tenderloin on all sides with the mustard. Pack the cooled mushroom mixture around the pork tenderloin. Place the coated meat on the outer third of the pastry. Brush a 1/2-inch border of egg wash on the pastry around the meat. Gently pull the remaining pastry over the meat to completely enclose, and press gently to seal. With a small knife cut away the excess pastry to make an even border and crimp the pastry edges with a fork dipped in flour.
Carefully transfer the tenderloin onto the prepared baking sheet. Brush the pastry evenly with egg wash. With a small knife make a decorative crosshatch pattern across the top. Bake for 10 minutes, then turn. Bake until golden brown and an instant read thermometer reaches 140 degrees F, 10 to 12 minutes. Remove from the oven and let rest for 5 minutes before serving.
Yield: 4 servings