In a skillet over medium heat, sauté finely chopped mushrooms for about 5–7 minutes until their moisture evaporates. Add cooked, seasoned beef tenderloin and combine well. Allow the filling to cool slightly.

Roll out cold puff pastry on a lightly floured surface until it's about 1/8-inch thick. Cut into 3 x 3 inch squares.

Place a spoonful of filling in the center of each pastry square. Brush edges with beaten egg and fold corners over filling, pinching to secure seams.

Place assembled bites on a parchment-lined baking sheet and refrigerate for at least 30 minutes.

Preheat oven to 400°F (200°C). Bake bites for 15–20 minutes or until golden brown.

Remove from oven and let cool for a few minutes. Garnish with fresh herbs and serve warm.