Heat a drizzle of olive oil in a large skillet over medium-high heat. Season beef cubes with salt and pepper and sear for 2-3 minutes on each side until golden-brown. Remove from heat and let cool.

In the same skillet, add finely chopped mushrooms, shallots, and garlic. Cook for 5-7 minutes until golden brown. Add thyme and let cool.

Lightly flour your work surface and roll out puff pastry sheets to 1/8 inch thick. Cut into 3x3 inch squares.

On each pastry square, add mushroom duxelles and a beef cube. Optionally brush with Dijon mustard. Fold pastry corners over filling and seal with a fork.

Preheat oven to 400°F (200°C). Arrange assembled bites on a baking sheet and brush with egg wash. Bake for 15-20 minutes until pastry is golden.

Allow bites to cool for a few minutes before serving. Enjoy warm for best flavor!