Heat a skillet over medium-high heat, add olive oil, season beef cubes, and sear for 2-3 minutes on each side until browned.

In the same skillet, reduce heat to medium, add mushrooms, shallots, and garlic. Sauté for 5-7 minutes until golden brown, then add thyme and season to taste.

Roll out the puff pastry into 1/8 inch thick squares of about 3x3 inches.

Place mushroom duxelles and a beef cube on the pastry square, optionally brush with Dijon mustard, fold, and seal the edges.

Preheat oven to 400°F (200°C), arrange bites on a parchment-lined baking sheet, brush with beaten egg and bake for 15-20 minutes.

Let the bites cool slightly on a wire rack before serving warm with dipping sauce.