Preheat oven to 375°.
Roll one pie crust out and place into a 12 inch cast iron skillet or deep baking dish. Prick the pie crust all over with a large tined fork. Place a large sheet of aluminum foil over the pie crust, pressing it down flat over the entire crust.
Bake for 15 minutes. Remove from oven and set aside, remove the foil.
Add olive oil to another large skillet and cook the onions, carrots and celery, over medium- high heat. Stir often until softened. Remove to a bowl and set aside.
In the same skillet, melt butter and whisk in the flour. Cook over medium heat, whisking until golden brown.
Add the beef broth and Worcestershire sauce, whisking until well combined.
Next, add the potatoes and continue to cook until gravy thickens.
Stir in the brisket, cooked vegetables and English peas.
Pour mixture into the prepared pie crust.
Roll out the top pie crust and cover the filling. Trim if needed and crimp the edges to seal. Cut 2- 3 slits in the top using a sharp knife.
Beat the egg and water together and brush over the pie crust.
Place the skillet onto a rimmed baking sheet to catch any spills if it cooks over. Bake for 45 minutes or until crust is golden brown and filling is bubbly.
Remove from oven and cool 10 minutes before slicing and serving.