In a large skillet over medium-high heat, melt the butter in a large frying pan. Add the bacon, onions, and mushrooms and cook for about 6-7 minutes, stirring occasionally, until fully cooked and golden. Add the garlic and cook one more minute.
Add 2 tablespoons of butter to the pan. Add the flour and stir to make a roux. Cook for 1 minute, then whisk in the chicken stock and the milk, until smooth. Whisk until the sauce thickens. Let the liquid simmer for a few seconds to thicken .
Stir in the mustard, along with salt and pepper. Add the shredded chicken to the skillet and stir until everything is fully coated. Remove from heat and let cool completely before pouring into the prepared pie crust.Preheat the oven to 400β.
Fill the pie crust and bake or freeze for later
Once the pie filling has cooled, place the bottom crust in a pie dish, then pour in the chicken filling.Place the top crust on top and seal the edges, pressing down gently with a fork.To freeze: wrap the pie in plastic wrap and foil and freeze at this stage. Refridgerate overnight in the fridge to thaw and bake as followed.
To Bake: Place a pie shield on the pie (or loosely cover with foil) and set on a baking sheet to catch any drippings.Bake at 400β for 50-60 minutes or until golden brown and cooked through. Let cool for 10-15 minutes before serving.