Heat a tablespoon of olive oil in a large skillet over medium-high heat. Season your beef cubes with salt and pepper, then add them to the skillet, searing for 2-3 minutes until browned on all sides, but not fully cooked. Remove the beef from heat and let it cool slightly on a plate while preparing the mushroom duxelles.

In the same skillet, add chopped mushrooms, shallots, and minced garlic. Cook over medium heat for 5-7 minutes, stirring occasionally, until the mixture is golden brown and most of the moisture has evaporated. Stir in fresh thyme, season with salt and pepper, and let the duxelles cool to room temperature.

On a lightly floured surface, roll out your thawed puff pastry to a thickness of 1/8 inch. Using a sharp knife or pizza cutter, cut the pastry into 3x3 inch squares. Keep the rolled pastry covered to prevent it from drying out as you assemble.

Take a square of puff pastry and place a teaspoon of mushroom duxelles at the center, followed by a piece of the seared beef. Optionally, spread a layer of Dijon mustard on the beef. Fold the pastry over the filling to create a triangle or rectangular pocket and seal the edges firmly with a fork.

Preheat your oven to 400°F (200°C). Place each bite on a lined baking sheet, brushing them generously with beaten egg. Bake in the oven for 15-20 minutes, or until the pastry is puffed and golden brown.

Once baked, remove the Mini Beef Wellington Bites from the oven and allow them to cool slightly on a wire rack. Serve warm.