Preheat oven to 375° F
Butter a 9" x 13" dish and add a layer of the defrosted diced hash brown potatoes
For the Filling: Melt 4 tbs of the butter in a large saucepan over medium heat. Add the carrots and cook for 5 minutes. Add the celery, onion and a pinch of salt and pepper; cook until the onions are soft and translucent (about 5 minutes)
Add the remaining 4 tbs butter and, once melted, add the flour (or gluten free flour) to make a roux. Cook the flour for 2 minutes, stirring constantly, then add the milk and chicken broth. Simmer, stirring frequently, until the mixture thickens, 5-7 minutes.
Stir in the chicken, corn, peas, celery seeds and garlic powder. Remove from the heat. Pour over the diced potatoes in the baking dish and spread into an even layer.
For the Crust: In a large mixing bowl, whisk together the baking mix (regular or gluten free) and pepper until well blended. Whisk in the melted butter and buttermilk until smooth. Stir in the parmesan. Pour the batter over the chicken filling in the baking dish. Spread evenly up to the edges. Be careful not to mix the crust into the filling.
Bake until the crust is crispy and golden brown, 40-45 minutes. Let sit for 10 minutes before serving. Enjoy!