Preheat & Prep
Preheat your oven to 425°F (220°C). Place one pie crust in a 9-inch pie dish, gently pressing it in. Keep the top crust chilled.
Make the Filling
In a large skillet or Dutch oven, melt butter over medium heat. Add onions, carrots, and celery; cook 5–7 minutes, until softened.
Sprinkle flour over veggies, stir to coat, and cook for 1–2 minutes. Slowly whisk in chicken broth and milk, stirring constantly until smooth and thickened (about 5–7 minutes). Season with salt, pepper, and garlic powder.
Stir in diced cooked chicken and peas. Remove from heat.
Assemble the Pie
Spoon the warm filling into the prepared pie crust. Top with the second crust, trim edges, and crimp to seal. Cut 4–6 small slits in the top crust to let steam escape. Brush with egg wash.
Bake
Place the pie on a baking sheet (to catch spills) and bake for 30–35 minutes, or until the crust is golden brown and filling is bubbling. If edges brown too quickly, cover them with strips of aluminum foil.
Rest & Serve
Let the pot pie cool for at least 10–15 minutes before slicing. This helps the filling set.