Cut the boneless, skinless chicken breasts into 1-1.5 inch pieces and season with salt and pepper. Let the chicken sit at room temperature for about 15 minutes.

In a large frying pan, melt 2 tablespoons of unsalted butter over medium-high heat. Add the bacon and cook until crispy, about 5-7 minutes. Remove the bacon and set aside.

Add the seasoned chicken to the same pan and cook until mostly cooked through, about 5-6 minutes. Remove the chicken and cover to keep warm.

In the same pan, add the remaining butter and let it melt. Toss in the mixed mushrooms and sauté until browned, approximately 5-7 minutes. Set mushrooms aside.

Sauté the chopped onion and minced garlic in the remaining fat until softened, about 3-4 minutes. Sprinkle in the flour, stir to coat, and cook for an additional 1-2 minutes. Gradually pour in the chicken stock, stirring until the sauce thickens, about 3-4 minutes.

Stir in the milk, Dijon mustard, dried thyme, cooked bacon, sautéed mushrooms, and warm chicken. Mix well and set aside to cool slightly.

Preheat your oven to 410°F (210°C). Line a 9-inch pie dish with parchment paper; place one sheet of puff pastry at the bottom. Pour in the filling, then cover with a second sheet of pastry. Seal the edges and trim any excess.

Brush the top of the pastry with an egg wash for a golden finish and cut a few slits to allow steam to escape. Bake for 20 minutes, then reduce temperature to 350°F (175°C) and bake for an additional 20 minutes or until golden brown.

Allow the pie to cool for about 15 minutes before slicing.