Preheat oven to 400°F (200°C).
Steam broccoli florets for 3 minutes until slightly tender but still firm; drain well.
In a large saucepan, melt butter over medium heat and sauté onion until translucent, approximately 5 minutes.
Whisk in flour and cook for 1 minute to create a roux.
Gradually whisk in chicken broth and milk, bringing to a simmer and whisking constantly until thickened.
Remove from heat and stir in salt, pepper, garlic powder, shredded chicken, steamed broccoli, and 1 cup of cheddar cheese.
Place one pie crust into a 9-inch pie plate and pour the chicken mixture into the crust.
Sprinkle the remaining 1/2 cup of cheese over the filling and top with the second pie crust.
Seal the edges and cut small slits in the top for steam to escape.
Bake for 30 to 35 minutes, or until the crust is golden brown, then let stand for 10 minutes before serving.