Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

Combine ground beef, chopped onion, Worcestershire sauce, garlic powder, salt, pepper, and breadcrumbs in a mixing bowl. Add beaten egg and mix well.

Shape the mixture into eight equal patties. Place a slice of provolone cheese on top of each patty.

Roll out chilled puff pastry and cut into four squares each; you will need eight squares total.

Place a beef patty in the center of each puff pastry square, fold corners over, and seal edges tightly.

Whisk together egg yolk and water for egg wash. Brush tops of pastries with the wash.

Bake for 25-30 minutes until golden and puffed. Monitor towards the end for desired color.

Allow to cool slightly on a wire rack. Optionally garnish with fresh herbs before serving.