Heat 1 tablespoon of olive oil in a skillet over medium-high heat. Season the beef cubes with salt and pepper, then add them to the pan. Sear the beef for about 2-3 minutes until browned on all sides. Remove from skillet and set aside to cool.

In the same skillet, add another tablespoon of olive oil and lower the heat to medium. Sauté finely chopped mushrooms, shallots, and minced garlic for 5-7 minutes until the mushrooms are golden and the shallots are translucent. Stir in fresh thyme, salt, and pepper, cook for an additional minute, then remove from heat and allow to cool.

On a lightly floured surface, roll out puff pastry sheets to about 1/8 inch thick. Cut into 3x3 inch squares and keep them on a parchment-lined surface.

Preheat your oven to 400°F (200°C) and prepare a baking sheet with parchment paper. Place a spoonful of the cooled mushroom mixture in the center of each pastry square, top with a cooled beef cube, and optionally spread a thin layer of Dijon mustard on the beef. Fold the pastry over the filling and seal using a fork.

Brush the tops with a beaten egg wash for a golden finish. Arrange on the baking sheet and bake for 15-20 minutes until puffed and golden brown.

Remove from the oven and let cool for a few minutes. Serve warm with a dipping sauce like horseradish or mustard.