Begin by rinsing and cutting boneless, skinless chicken breasts into 1 to 1.5-inch chunks. Season with salt and pepper.

In a large skillet, cook chopped bacon over medium heat until crispy, about 5-7 minutes. Set aside, leaving drippings in the pan.

In the same skillet, add seasoned chicken pieces and cook until mostly cooked through, about 5-6 minutes. Remove and set aside.

Add sliced mushrooms to the skillet and sauté until browned, about 4-5 minutes. Remove and keep warm.

Lower the heat, add diced onion and minced garlic, cooking until translucent, about 2-3 minutes. Stir in flour and cook for 1 minute.

Gradually whisk in chicken stock, cooking until thickened, about 3-4 minutes. Stir in milk, Dijon mustard, and dried thyme.

Add cooked chicken, bacon, and mushrooms to the creamy sauce and stir gently.

Preheat oven to 410°F (210°C). Line a pie dish with parchment paper, roll out the first sheet of puff pastry, and spoon in the filling.

Place a second sheet of puff pastry over the top, sealing the edges. Bake for 20 minutes, then reduce to 350°F (180°C) and bake for another 20 minutes.

Allow to cool for a few minutes before serving.