In a large skillet, melt the butter over medium heat. Add the diced onion, carrots, and celery, and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute.

Add the cubed beef to the skillet and cook until browned on all sides. Sprinkle the flour over the beef and vegetables, stirring to coat evenly. Pour in the beef broth and grape juice, and stir to combine. Add the thyme, rosemary, salt, and pepper. Bring the mixture to a simmer and cook for 15-20 minutes, or until the beef is tender and the sauce has thickened.

Preheat your oven to 375°F (190°C). Divide the beef filling evenly among four ramekins or small oven-safe dishes. Cut the puff pastry into circles slightly larger than the ramekins and place them on top of the filling. Press the edges to seal and brush the tops with the beaten egg.

Place the ramekins on a baking sheet and bake for 20-25 minutes, or until the pastry is golden brown and puffed. Let the pot pies cool for a few minutes before serving.