Begin by finely chopping the mushrooms and sautéing them in a skillet over medium heat for about 5–7 minutes, or until all moisture has evaporated.

Combine the sautéed mushrooms with the cooked beef tenderloin in a bowl, mixing well and seasoning with salt, pepper, and Dijon mustard.

Roll out the cold puff pastry on a lightly floured surface until about 1/8 inch thick and cut into squares or rectangles of approximately 3x3 inches.

Place a spoonful of the filling in the center of each pastry square, ensuring not to overfill.

Fold the pastry over the filling to create a pocket, sealing the edges tightly by crimping them with a fork.

Brush the tops with egg wash to achieve a golden finish when baked.

Refrigerate the assembled bites for 20 minutes to firm up the pastry and filling.

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

Remove from the oven and let cool for a couple of minutes before serving.

Arrange on a platter and garnish with fresh herbs or a side of your favorite dipping sauce.