In a large bowl, toss the cubed beef with flour, salt, and black pepper until evenly coated.

Heat olive oil in a large skillet over medium-high heat. Add the beef and sear until browned on all sides. Remove the beef from the skillet and set it aside.

In the same skillet, add the diced onion, sliced carrots, and celery. Cook until the vegetables are softened, about 5 minutes. Add the minced garlic and cook for another minute.

Pour in the beef broth and grape juice, scraping the bottom of the skillet to release any browned bits. Stir in the tomato paste, thyme, and rosemary. Return the beef to the skillet and bring the mixture to a simmer. Reduce the heat to low, cover, and let it cook for about 1 hour, or until the beef is tender.

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

Preheat your oven to 375°F (190°C). Transfer the beef filling to a large 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 create 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.