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 ⅛-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.