Begin by heating 1 tablespoon of olive oil in a large skillet over medium-high heat. Season the tender beef cubes with salt and pepper, then carefully add them to the hot skillet. Sear the beef for about 2-3 minutes, or until nicely browned on all sides. Once seared, remove the beef from the skillet and set aside to cool.

In the same skillet, add another tablespoon of olive oil and turn the heat down to medium. Add chopped mushrooms, minced shallots, and minced garlic, stirring occasionally. Cook for approximately 5-7 minutes, allowing the mixture to cook down until all excess moisture evaporates and the mushrooms are golden brown. Stir in fresh thyme and seasoning, then let it cool.

While the mushroom duxelles cools, take your puff pastry out of the freezer to thaw. Once thawed, lightly flour your work surface and roll out the pastry to about 1/8 inch thick. Use a sharp knife or pizza cutter to slice the pastry into 3x3 inch squares. Keep the squares cool by placing them on a baking sheet in the refrigerator until ready for assembly.

Take a pastry square and spoon a small amount of the mushroom duxelles into the center, followed by a piece of the seared beef. If you desire, brush a thin layer of Dijon mustard over the beef. Carefully fold the pastry over the filling and pinch the edges to seal them tightly, being sure to press out any air pockets. Repeat until all ingredients are used.

Preheat your oven to 400°F (200°C) while assembling the Mini Beef Wellington Bites. Line a baking sheet with parchment paper. Arrange the assembled bites on the prepared baking sheet. Brush the tops with a beaten egg for a golden finish. Bake in the preheated oven for 15-20 minutes, or until the pastry is puffed and golden brown.

Once the Mini Beef Wellington Bites are baked to perfection, remove them from the oven and let them cool for a few minutes on a wire rack. Serve warm alongside creamy horseradish sauce or tangy mustard.