Start by finely chopping the mushrooms and sautéing them in a skillet over medium heat for about 5-7 minutes, until all moisture evaporates. Add in the chopped beef tenderloin and cook for another 3 minutes, stirring occasionally until the beef is browned but not fully cooked. Remove from heat and mix in a tablespoon of Dijon mustard for an added tangy flavor.

Roll out the puff pastry on a lightly floured surface to about 1/8 inch thick. Cut the pastry into squares, approximately 3x3 inches in size. Place a spoonful of the beef-mushroom mixture in the center of each square.

Fold the pastry over the filling to create a triangle or rectangle shape, pressing the edges to seal tightly. Use a fork to crimp the edges for added security and a decorative touch.

Prepare an egg wash by whisking one egg and brush it generously over the tops of the pastries.

Place the assembled Mini Beef Wellington Bites on a parchment-lined baking sheet. Refrigerate them for at least 30 minutes.

Preheat your oven to 350°F (175°C). Once preheated, bake the bites for 15-20 minutes or until they puff up nicely and turn a rich golden brown.

After removing the bites from the oven, let them cool for a few minutes on the baking sheet. Serve warm on a platter, garnished with fresh herbs like parsley or thyme.