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

In the same skillet, add mushrooms, shallots, and garlic. Cook over medium heat for 5-7 minutes until moisture is released and mushrooms are golden. Stir in thyme and season with salt and pepper. Let cool.

Roll out puff pastry on a floured surface to about 1/8 inch thick. Cut into 3x3 inch squares.

Place a pastry square on the workspace, add a spoonful of the mushroom mixture and a beef cube in the center. Optionally brush with mustard, then fold and seal the pastry.

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

Remove from oven and cool slightly on a wire rack before serving, best enjoyed warm.