Heat olive oil in a skillet over medium-high heat. Season the beef cubes with salt and pepper, then sear for 2-3 minutes on each side until browned. Remove and let cool.

In the same skillet, add chopped mushrooms, shallots, and garlic, cooking over medium heat for 5-7 minutes until golden. Stir in thyme, season, and cool.

Roll puff pastry sheets to 1/8 inch thick on a floured surface. Cut into 3x3 inch squares.

Place a spoonful of mushroom duxelles in each pastry square center, top with seared beef, and optionally spread Dijon mustard. Fold pastry corners over filling and seal edges.

Preheat oven to 400°F (200°C). Arrange bites on a parchment-lined baking sheet, brush with beaten egg, and bake for 15-20 minutes until golden.

Allow to cool slightly before serving warm. Enjoy!