First, prepare the pie crust according to the recipe instructions.
On a lightly floured piece of parchment paper, roll out the dough into a rectangle about 1/8-inch thick. Place the rolled-out dough, still on the paper, in the refrigerator to chill while you make the filling.
Preheat your oven to 375°F. Grease a round ovenproof baking dish at least 10 inches in diameter, and set it aside. Alternatively, grease 4-oven proof baking dishes at least 6 inches in diameter each.
In a large saucepan, melt the butter over medium heat. Add the olive oil, and the onions, and mix to combine and coat the onions in the oil and butter.
Add the salt and pepper, and mix to combine. Sauté the onions, stirring occasionally, until translucent but not browned, about 6 minutes.
Add the flour slowly, mixing constantly, until the flour is well incorporated. Continue to cook for about 2 minutes or until the mixture begins to smell a bit nutty.
Add the chicken stock and then the milk or cream in a slow, steady stream, mixing constantly.
Raise the heat to medium high and cook, stirring occasionally, until the mixture begins to simmer and thicken. Once the mixture is simmering, add the thawed mixed vegetables and cooked chicken, and mix to combine.
Fill the prepared baking dish(es) with filling.
Remove the pastry dough from the refrigerator. Turn it over, and place it on top of the filling in the baking dish(es).
Gently remove the parchment paper from the top, and crimp or pinch the edges to secure. Trim excess pie crust using a sharp knife or kitchen shears.
Using a sharp knife, carefully slice an X in the middle of the pie(s) to allow steam to escape during baking. With a pastry brush, brush the top of the crust(s) with the egg wash.
Place the baking dish(es) on a rimmed baking sheet to catch any overflow or drips. Place in the center of the preheated oven and bake for about 35 minutes, or until the top is lightly golden brown all over. Serve immediately.
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Notes
For the pie crust.You can use whatever gluten free pie crust you like best, but the extra flaky pie crust recipe is my favorite, by far. It’s the lightest and flakiest, and browns quickly and beautifully.For dairy-free tips on that recipe, please click through to that post.For the chicken.You can use any cooked chicken you like that has been seasoned plainly, preferably with just salt, pepper, and a bit of olive oil. You can even use rotisserie chicken, or some of the plain roasted chicken breasts from Trader Joe’s.My favorite way to cook it for a recipe like this is to place the breasts in a casserole dish, with space between the breasts. Drizzle the meat lightly with olive oil, sprinkle lightly with salt and freshly ground black pepper.Cover the meat with parchment paper placed directly on top of it, covering it completely. Place in a 400°F oven and cook for about 20 minutes or until an instant-reader thermometer inserted into the thickest portion of each breast reads at least 165°F. Allow to rest outside the oven, still covered, for 10 minutes before chopping.To make this recipe ahead.You can make the entire pie, assembled and topped with crust, ahead of time. Just allow the pie to sit until the filling has cooled before covering with the pie crust. Place the baking dish in the freezer, uncovered, until the crust is frozen.Cover tightly, and return to the freezer. Defrost overnight in the refrigerator before baking as instructed.