Finely chop the mushrooms and heat a skillet over medium heat. Add the mushrooms and sauté for about 5-7 minutes until most of the moisture evaporates.

In a mixing bowl, combine the sautéed mushrooms with diced beef tenderloin and a generous dollop of Dijon mustard. Mix until well incorporated.

Preheat your oven to 350°F (175°C).

Roll out your cold puff pastry on a floured surface and cut it into 3-inch squares. Spoon a generous amount of the filling into the center of each square.

Transfer the assembled bites to a parchment-lined baking sheet and place them in the refrigerator for 30 minutes.

Brush each Wellington bite with an egg wash made from beaten egg for a shiny finish. Bake for 15-20 minutes until golden brown.

Allow the bites to cool slightly before serving.