Preheat oven to 425 degrees F.
In a large saucepan, cook diced onion in the butter over medium heat until translucent (about 5 minutes). Stir in gluten free flour, salt, pepper and garlic powder.
Slowly stir into the large saucepan, chicken broth (or water) and milk. Simmer over medium low heat until thick, then remove from heat.
Add into the onion and cream mixture, the frozen veggies and the shredded rotisserie chicken. Mix together until spread all throughout the cream.
Place one of the GF pie crusts in the bottom of a pie pan. Pour the hot cream/onion mixture with chicken and veggies into the pie crust. Cover with the second GF crust on top. Seal edges and cut away excess dough. Make a few small slits in the top of the pie crust to allow steam to escape.
Bake in preheated oven for 30-35 minutes, until crust is golden brown. Cool for 10 minutes before serving.