Finely chop the mushrooms and sauté them in olive oil for 8–10 minutes until golden brown.

Let the mushrooms cool, season with salt, pepper, and Worcestershire sauce if desired.

Roll out the puff pastry to 1/8 inch thick and cut into 3-inch squares.

In the center of each square, place a spoonful of the mushroom mixture and a piece of beef. Add a dab of Dijon mustard and fold the pastry, sealing the edges.

Refrigerate the assembled bites for 30 minutes to maintain shape.

Preheat oven to 400°F (200°C). Beat an egg for the egg wash and brush each bite.

Bake in the preheated oven for 15–20 minutes until golden and puffed.

Cool on a wire rack before transferring to a platter. Serve warm.