Thaw pie shell for 10 minutes. Preheat oven to 400°F.
Remove protective sheet from pie shells before preparing.
Microwave the frozen mixed vegetables for a couple of minutes or sauté onions in a little vegetable oil, add carrots and peas and remove from heat as soon as they soften slightly.
In a saucepan, over medium heat, melt butter and add gluten free flour and cornstarch. Whisk in milk, chicken broth and seasonings. Stir while cooking over medium heat until it comes to a boil and thickens.
In a bowl, mix together chicken, vegetables and sauce.
Peel potatoes, cut into pieces and boil until tender
Drain water, mash the potatoes and add butter, milk, and seasoning.
Place pie shell on a baking sheet. Pour filling into the pie shell and top with an even layer of mashed potatoes.
Cover edge with metal collar or strips of aluminum foil. Bake on the center rack in the preheated oven for approximately 40 minutes.
Serve or refrigerate for up to 3 days.