Rinse and pat dry the chicken; cut into cubes, season with salt and pepper, and set aside.

In a skillet, melt butter and cook bacon until crispy, then set aside on paper towels.

Sauté the seasoned chicken in the bacon fat over medium-high heat until golden and nearly cooked through, then remove from skillet.

Sauté mushrooms in remaining butter until browned and moisture evaporates; set aside.

Sauté diced onion and minced garlic until onion is translucent; then add flour and cook for an additional minute.

Gradually mix in chicken stock, stirring until thick; then add milk, Dijon mustard, and thyme.

Combine chicken, bacon, and mushrooms with the creamy filling until evenly coated.

Preheat oven to 410°F (210°C).

Roll out one sheet of puff pastry, line a pie dish, pour in the filling, and cover with a second pastry sheet; seal edges and brush with egg wash.

Bake at 410°F (210°C) for 20 minutes, then reduce to 350°F (180°C) and bake for another 20-30 minutes until golden brown.

Let cool for 15 minutes before serving.