In a large bowl, toss the cubed beef with flour, salt, and black pepper until evenly coated. Heat olive oil in a large pot over medium heat. Add the beef and cook until browned on all sides. Remove the beef from the pot and set it aside.

In the same pot, add the diced onion, sliced carrots, chopped celery, and minced garlic. Cook until the vegetables are softened, about 5 minutes.

Return the beef to the pot. Stir in the beef broth, grape juice, tomato paste, thyme, and rosemary. Bring the mixture to a boil, then reduce the heat to low and let it simmer for 45 minutes, or until the beef is tender.

Stir in the frozen peas and cook for an additional 5 minutes. Remove the pot from the heat and let the filling cool slightly.

Preheat your oven to 375°F (190°C). Transfer the beef filling to a 9-inch pie dish. Roll out the puff pastry sheet and place it over the filling, trimming any excess dough. Brush the pastry with the beaten egg for a golden finish.

Bake the pot pie in the preheated oven for 25-30 minutes, or until the crust is golden brown and flaky. Let it cool for a few minutes before serving.