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-high heat. Add the beef in batches and sear until browned on all sides. Remove the beef from the pot and set it aside.

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

Pour in the beef broth and grape juice, scraping the bottom of the pot to release any browned bits. Stir in the tomato paste, thyme, and rosemary. Return the beef to the pot and bring the mixture to a simmer. Reduce the heat to low, cover, and let it cook for 1.5 hours, 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 deep 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 to give it 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.