Heat the olive oil in a large pot over medium-high heat. Add the cubed beef 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, sliced carrots, and celery. Cook until the vegetables are softened, about 5 minutes. Add the minced garlic and cook for another minute.

Sprinkle the flour over the vegetables and stir to combine. Cook for 1-2 minutes to remove the raw flour taste.

Pour in the beef broth and grape juice, stirring to combine. Add the tomato paste, thyme, rosemary, bay leaf, salt, and pepper. Return the beef to the pot and bring the mixture to a simmer.

Reduce the heat to low, cover the pot, and let the stew simmer for 1.5 to 2 hours, or until the beef is tender and the flavors have melded together.

Preheat your oven to 375°F (190°C). Transfer the beef stew to a large baking dish. Roll out the puff pastry and place it over the top of the dish. Trim any excess pastry and crimp the edges to seal. Brush the top 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.