Heat 2 tablespoons of olive oil in a skillet over medium-high heat. Season beef cubes with salt and pepper, then add to the hot skillet. Sear for 2-3 minutes until browned but not fully cooked, then remove and let cool.

In the same skillet, add another tablespoon of olive oil and sauté 8 ounces of chopped mushrooms, 2 minced shallots, and 2 cloves of minced garlic for 5-7 minutes until golden and moisture evaporates. Stir in thyme, season, then cool.

Roll out thawed puff pastry to about 1/8-inch thick. Cut into 3x3 inch squares (around 12 pieces total), keeping squares covered with a towel.

Take each pastry square and place a spoonful of mushroom duxelles and a piece of seared beef in the center. Brush with Dijon mustard, fold over, and seal edges to avoid air pockets.

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

Remove from oven and let cool for a few minutes. Serve warm with dipping sauce such as horseradish or mustard.