Begin by heating 1 tablespoon of olive oil in a large skillet over medium-high heat. Season your beef cubes with salt and pepper and sear for 2-3 minutes until nicely browned. Remove from skillet and let cool.

In the same skillet, add finely chopped mushrooms, shallots, and minced garlic. Sauté over medium heat for 5-7 minutes until golden brown. Add thyme, salt, and pepper, then transfer to a bowl and cool.

On a lightly floured surface, roll out the puff pastry into a rectangle until 1/8 inch thick. Cut into 3x3 inch squares and cover with a damp cloth.

Take each square of pastry, add a spoonful of cooled mushroom duxelles, place a piece of seared beef on top, brush with Dijon mustard, and fold pastry over filling, sealing the edges.

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

Once baked, let cool slightly on a wire rack. Serve warm with dipping sauces like horseradish or mustard.