Finely chop mushrooms and set aside. In a skillet over medium heat, brown beef tenderloin for about 3-4 minutes. Mix in Dijon mustard and let cool.

Roll out the chilled puff pastry on a lightly floured surface until 1/8-inch thick. Cut into 3-inch squares.

Place about a tablespoon of the beef-mushroom mixture in the center of each square. Fold corners and seal edges tightly with a fork.

Refrigerate assembled bites on a parchment-lined baking sheet for at least 30 minutes.

Preheat oven to 400°F (200°C). Brush tops with egg wash and bake for 15-20 minutes until golden brown.

Allow to cool slightly on a wire rack before serving warm.