Heat a tablespoon of olive oil in a large skillet over medium-high heat. Season beef with salt and pepper, add to the skillet, and sear for 2-3 minutes until browned. Remove and let cool.

In the same skillet, add chopped mushrooms, shallots, and minced garlic. Cook over medium heat for 5-7 minutes until golden brown and moisture has evaporated. Add thyme and season to taste, then remove from heat.

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

Place a tablespoon of mushroom duxelles in the center of each pastry square, followed by a piece of seared beef. Optionally smear with Dijon mustard, fold the pastry over, and seal edges with a fork.

Preheat oven to 400°F (200°C) and prepare a baking sheet with parchment paper.

Arrange assembled bites on the baking sheet, brush with beaten egg, and bake for 15-20 minutes until golden brown and puffed.

Cool on the baking sheet for a few minutes before serving warm.