Step 1: Preheat your oven to 400°F (200°C).
Step 2: In a large skillet, heat the olive oil over medium heat. Add the diced onion, carrots, and celery. Sauté for 5-7 minutes until vegetables are softened.
Step 3: Stir in the minced garlic and cook for an additional minute until fragrant.
Step 4: Add the shredded or chopped brisket, beef broth, Worcestershire sauce, dried thyme, dried rosemary, salt, and pepper to the skillet. Stir to combine.
Step 5: If you desire a thicker filling, mix the cornstarch with 2 tablespoons of cold water in a small bowl to create a slurry. Stir the slurry into the brisket mixture and cook, stirring, until the filling thickens slightly.
Step 6: Stir in the frozen peas and remove the skillet from heat. Allow the filling to cool slightly while you prepare the crust.
Step 7: Roll out one of the chilled pie crusts and carefully fit it into a 9-inch pie dish. Trim any excess dough around the edges.
Step 8: Pour the cooled brisket filling evenly into the pie crust-lined dish.
Step 9: Roll out the second pie crust and place it over the filling. Trim the edges and crimp them together with the bottom crust to seal. Cut several small slits into the top crust to allow steam to escape during baking.
Step 10: In a small bowl, lightly beat one egg. Using a pastry brush, evenly brush the beaten egg over the entire top surface of the pie crust. This will give it a beautiful golden finish.
Step 11: Place the assembled pot pie in the preheated oven and bake for 25-30 minutes, or until the crust is golden brown and the filling is bubbling hot.
Step 12: Once baked, carefully remove the pot pie from the oven. Let it cool for about 10-15 minutes before slicing and serving. This resting period helps the filling set, making for cleaner slices. Enjoy your hearty brisket pot pie!