Heat the olive oil in a large skillet over medium heat. Add the cubed beef and cook 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, chopped celery, and minced garlic. Cook until the vegetables are softened, about 5 minutes.

Sprinkle the flour over the vegetables and stir to coat. Gradually pour in the beef broth, stirring constantly to avoid lumps. Bring the mixture to a simmer and cook until it thickens.

Return the cooked beef to the skillet and add the frozen peas, dried thyme, dried rosemary, salt, and pepper. Stir well to combine and let the mixture simmer for another 5 minutes.

Transfer the beef and vegetable mixture to a pie dish. Roll out the puff pastry sheet and place it over the filling, trimming any excess dough. Brush the top of the pastry with the beaten egg to give it a golden finish.

Preheat your oven to 375°F (190°C). Bake the pot pie for 25-30 minutes, or until the crust is golden brown and flaky.