Heat a skillet over medium-high heat and add olive oil. Season beef cubes with salt and pepper, then sear them for 2-3 minutes per side until brown.

In the same skillet, add chopped mushrooms, shallots, and garlic. Cook for 5-7 minutes until golden brown and moisture has evaporated. Stir in fresh thyme and let cool.

Roll puff pastry on a floured surface to 1/8 inch thick. Cut into 3x3 inch squares and keep cold on a baking sheet.

Place a spoonful of mushroom duxelles on a pastry square, add seared beef, and brush with Dijon mustard. Fold pastry over filling and seal edges with a fork.

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