Finely chop the mushrooms and heat a skillet over medium heat. Sauté the mushrooms for about 5-7 minutes, until golden brown. Combine with cooked and diced beef tenderloin and Dijon mustard. Let cool.

Roll out the pastry on a lightly floured surface to about 1/8-inch thickness. Cut the pastry into squares, approximately 3 inches by 3 inches.

Place a spoonful of the filling in the center of each pastry square. Fold the pastry over to form a triangle or rectangle. Pinch tightly to seal.

Cover the baking sheet with plastic wrap and chill the assembled bites for at least 30 minutes.

Preheat your oven to 400°F (200°C). Prepare an egg wash by whisking together an egg with a tablespoon of water.

Brush the tops of each bite with the egg wash. Bake for 15-20 minutes, or until golden brown.

Once baked, let the bites cool for a few minutes. Transfer to a serving platter and serve warm with your favorite drinks.