When you are are ready to assemble, start by beating an egg in a small bowl.
Put your beef filet on a cutting board and slice into ¼ inch thick slices. It's ok if it looks really pink or even rare in the center as the beef will cook inside the pastry.
Using a large cutting board, sprinkle with flour. Carefully open up a puff pastry sheet and place on the floured board. Roll it out evenly so that it grows in size about an inch in every direction. Using a sharp knife or a pizza cutter, cut the pastry into 10 even rectangles.
Using a pastry brush or your finger if you don't have one, brush the egg mixture along the edges of each pastry rectangle.
Place a slice of beef in the center of the rectangle, then scoop a small amount of the mushroom mixture on top. Fold the sides of the pastry over and press to seal. Then turn the beef 'package' over so the seal is on the bottom.
Using a fork, press the sides together to seal the sides.
Place all of the mini beef wellington bites on a large sheet pan and brush with the remaining egg mixture. At this point, you can place the sheet pan in the freezer and then transfer them to a freezer container once frozen. They will last in the freezer for 3 months.
Bake in the oven on the center (and top rack if you have 2 pans) for 10-12 minutes or until the pastry has puffed and the tops are golden brown. (if you are baking from frozen state, add about 5 minutes to the cooking time, but DO NOT THAW BEFORE COOKING)
Serve and Enjoy!