Start by seasoning and quickly searing the beef cubes in a hot skillet. Cook just until browned outside to lock in juices without overcooking. Set aside and let it cool before assembling.
In the same pan, melt butter and sauté shallots, garlic, and mushrooms. Cook until all moisture evaporates and mixture becomes thick and fragrant. Add thyme, salt, and pepper, then let it cool completely.
Roll out puff pastry and cut into small squares for bite-sized portions. Layer mushroom mixture, beef, and a touch of Dijon mustard. Wrap and seal edges carefully so filling stays inside while baking.
Brush each bite with egg wash for a shiny golden finish. Bake in a preheated oven until puff pastry is crisp and puffed. Let them cool slightly before serving for best texture and flavor.