Preheat your oven to 400°F (200°C). Finely chop your mushrooms and sauté in a skillet over medium heat for about 5 minutes. Combine the mushrooms with seasoned beef tenderloin and Dijon mustard.

Lightly flour your work surface and roll out the puff pastry until 1/8 inch thick. Cut squares each around 3 inches on each side. Place filling in the center and brush edges with egg wash to seal.

Fold the pastry over the filling to create sealed pockets. Arrange on a baking sheet, leaving space for even cooking.

Brush tops with egg wash. Chill in the fridge for 15 minutes, then bake for 15 minutes or until golden brown.

Serve warm on a platter, garnished with fresh herbs.