Preheat your oven to 375°F (190°C).

Prepare the pie crust by letting it come to room temperature if refrigerated.

Shred the cooked chicken and measure out the mixed vegetables and cheese.

Beat the egg in a small bowl and set aside for the egg wash.

In a large skillet, melt the butter over medium heat. Whisk in the flour and cook for about 1-2 minutes until lightly golden to form a roux.

Gradually whisk in the chicken broth and milk, stirring constantly until the sauce thickens, about 5-7 minutes.

Add garlic powder, onion powder, salt, and pepper to the sauce. Stir in the shredded chicken, mixed vegetables, and half of the shredded cheese until well combined.

Transfer the chicken mixture into a greased 9x13 inch casserole dish. Sprinkle the remaining cheese evenly over the top.

Lay the pie crust or puff pastry over the filling, trimming any excess and pressing edges to seal. Brush the crust with the beaten egg for a golden finish.

Bake the casserole for 30-35 minutes or until the crust is golden brown and the filling is bubbly. Let it cool for 5 minutes before serving.