Step 1: Heat a splash of olive oil in a skillet over medium-high heat. Sear beef cubes for 2-3 minutes on each side until browned, then let cool slightly.

Step 2: In the same skillet, add mushrooms, shallots, and garlic. Cook over medium heat for 5-7 minutes until golden brown. Mix in thyme, and let cool.

Step 3: Roll out the puff pastry on a floured surface to about 1/8 inch thick, then cut into 3x3 inch squares.

Step 4: Place mushroom duxelles and a piece of beef in the center of each pastry square. Brush with Dijon mustard if desired, fold and seal edges.

Step 5: Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper and arrange bites evenly spaced.

Step 6: Brush tops with beaten egg and bake for 15-20 minutes until pastry is puffed and golden brown.

Step 7: Allow to cool for a few minutes before serving with dipping sauce.