Start by putting the frozen puff pastry into the fridge to thaw the day before.

Preheat oven to 375 F degrees.

Finely Dice the Onion. Mince the garlic. You can store these in the fridge together until ready to cook.

You can make the beef a day or 2 early as well. You will be par-cooking the beef as it will finish cooking in the pastry later. Season very simply by rubbing olive oil all over and sprinkling liberally with salt and pepper.

Then I sear the outside of the filets in a pan on high heat for about 5 minutes on each side to get a nice brown crust.  Then I move it to the oven, center rack and cook for 15 minutes in 375 F degrees. When the beef is done cooking, transfer it to a plate to rest and cool down.  You can store covered in the fridge until ready to slice and assemble.

While the beef is cooking, you can make the Mushroom filling. Start by chopping the mushrooms into small pea-size pieces.  You can do this by hand, but it is much easier in a food processor. 

Add the mushrooms to a preheated skillet or frying pan with a teaspoon of olive oil (you can use the same one the beef cooked in).  Add the diced onions, minced garlic, salt, pepper, and oregano. Mix to combine and spread out into a layer covering the entire pan. Let cook without touching for 5 minutes. Then mix and spread into a layer and cook for another 5 minutes. When the mixture has cooked down and the onions are translucent, transfer it to a bowl to cool for a few minutes.

Add milk (or cream) and the parmesan cheese to the mushrooms and quickly mix through so it doesn't start to melt and clump. Store in the fridge to cool until ready to assemble.

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!