In a skillet over medium-high heat, heat a drizzle of olive oil. Sear the beef for 2-3 minutes per side until browned. Remove from pan and let cool, then slice into small pieces.

In the same skillet, add more oil if needed, and lower the heat. Sauté mushrooms, shallots, and garlic for 5-7 minutes until golden. Stir in thyme and cool.

On a floured surface, roll out the puff pastry to 1/8 inch thick. Cut into 3x3 inch squares.

Place a spoonful of cooled mushroom mixture in the center of each square, add a piece of beef and a dab of mustard. Fold corners over filling and seal with a fork.

Preheat oven to 400°F (200°C). Line a baking sheet with parchment and arrange bites. Brush with beaten egg. Bake for 15-20 minutes until golden.

Cool slightly before serving warm on a platter for the best flavor experience.