Preheat oven to 400 degrees F. Line two large baking sheets with parchment paper.
Heat oil in a large skillet over medium-high heat. Pat meat dry with paper towels. Season all over with salt and pepper. Add to skillet and sear all over, until browned, about 2-3 minutes. Remove from heat and clean skillet.
Melt butter in the same skillet over medium heat. Add mushrooms and shallots and sauté until tender. Season with salt and pepper. Stir in rosemary and remove from heat.
On a lightly floured surface, unfold puff pastry. Roll each sheet into a large square, about 1/8″-thick. Cut each into 16 squares. Place one piece of meat onto each square and dollop with about a teaspoon of mustard. Top the meat with about a tablespoon of the mushroom mixture. Fold pastry edges over the meat and pinch to seal. Place on baking sheets and brush with egg wash. Bake 14-16 minutes or until golden-brown.
Lamb Wellington https://westforkfarms.com/recipes/lamb-wellington February 29, 2020