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

In the same skillet, add more oil if needed and sauté mushrooms, shallots, and garlic over medium heat for 5-7 minutes until golden. Add thyme in the last minute, mix, and cool completely.

Roll out puff pastry on a floured surface until 1/8 inch thick. Slice into 3x3-inch squares. Keep covered to prevent drying while assembling.

Place mushroom duxelles in the center of each pastry square. Optionally brush beef with Dijon mustard before placing it on the mushrooms. Fold pastry over filling and seal edges.

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

Let cool slightly before serving warm with your favorite dipping sauces.