Preheat the oven to 400°F.
In a skillet over medium heat, add olive oil and sauté the chopped onion until golden brown, about 5-7 minutes.
Stir in garlic powder, onion powder, salt, and pepper. Remove from heat and let cool slightly.
Lay the flank steak slices flat on a clean surface. Spread the sautéed onions evenly over the steak.
Sprinkle mozzarella cheese and chopped parsley on top. Drizzle with balsamic vinegar and Worcestershire sauce.
Starting from one end, tightly roll the steak into a pinwheel shape. Secure with toothpicks if necessary. Cut the rolled steak into 1-inch thick slices.
Roll out the puff pastry on a lightly floured surface. Cut into squares large enough to wrap around each steak pinwheel.
Place a pinwheel in the center of each pastry square and fold the pastry over to enclose it completely.
Place the wrapped pinwheels on a baking sheet lined with parchment paper.
Bake for 20-25 minutes or until the pastry is golden brown and flaky.
Let cool for a few minutes before serving. Enjoy warm.