To a pot, melt butter and add flour, whisk to combine and cook until it darkens in color. Then add warm chicken broth whisking as you add it to prevent lumps. Stir frequently and cook until thickened, about 15 mins. Set aside.
To a saute pan, add olive oil and diced onion. Saute until translucent, then add frozen vegetables, season with salt and pepper and saute until tender. Add the gravy in with the vegetables.
Dice up the potato and add it to the mixture. Mix well to combine.
Preheat the oven to 400℉
Cut the pie dough in half. Roll out 1 piece to the size of a 9×12 pan. Place the bottom crust into the pan. Cut any excess and patch areas that need more dough.
Roll out the other piece of dough and set aside.
Add the filling into the pan with the bottom crust. Using a rolling pin, roll up the top crust and use it to guide the top crust over the filling. Fold the edges over and make sure to seal the seams well. Cut 4 slits into the top crust and brush with egg wash.
Bake for 45-1 hour until golden brown and bubbling.