Heat 1 tablespoon of olive oil in a skillet over medium-high heat. Season the beef cubes generously with salt and pepper, then sear them for about 2-3 minutes on each side until nicely browned. Remove from heat and allow to cool.

In the same skillet, add another tablespoon of olive oil if needed, then add the chopped mushrooms, shallots, and minced garlic. Cook for 5-7 minutes until golden brown and moisture is released. Stir in thyme and let the mixture cool.

On a floured surface, roll out the puff pastry until about 1/8 inch thick. Cut into 3x3 inch squares.

Place a small spoonful of mushroom duxelles at the center of each pastry square, followed by a piece of seared beef. Optionally brush a little Dijon mustard on top. Fold the pastry over to enclose the filling and seal the edges well.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Arrange the assembled bites on the sheet and brush the tops with beaten egg. Bake for 15-20 minutes until puffed and golden brown.

Remove from the oven and let cool for a few minutes. Serve warm on a platter with your favorite dipping sauce.