Preheat oven to 350℉.
Spray an oven safe casserole dish or deep dish pie pan liberally with cooking spray. Set aside.
Wash chicken breasts under cold running water and pat dry with a paper towel.
Cut each breast into bite sized pieces, about 1 inch chunks.
Season with salt and pepper. Set aside.
Cut bacon into ½ inch pieces.
Cook bacon in a large skillet over medium high heat until lightly browned and crisp.
Remove bacon from the skillet and drain on a paper towel lined plate.
Add chicken to the skillet and cook in the bacon fat until cooked through, about 7-8 minutes.
Remove cooked chicken from the skillet and cover with foil to keep warm.
Add 2 tablespoons butter to the skillet.
Cook mushrooms until they release their liquid and begin to brown along the edges.
Remove from skillet. Set aside.
Add another tablespoon of butter to the skillet and sauté minced onion until soft, about 4-5 minutes.
Add minced garlic and sauté, stirring constantly, until fragrant, about 1 minute.
Add another tablespoon of butter to the skillet.
Once butter is melted sprinkle flour into the pan.
Stir to coat the onions and garlic with the flour. Cook for about a minute to cook off the raw flour taste.
Start pouring in the chicken broth, a little at a time, stirring it to combine with the floured onions and garlic.
Add the milk, a little at a time, whisking continuously until sauce thickens.
Stir in Dijon mustard and thyme. Stir until incorporated. Sauce should be quite thick.
Reduce heat and stir in reserved bacon, mushrooms, and chicken pieces. Remove the pan from heat. Set aside.
Roll out a sheet of puff pastry dough on a floured work surface.
Place puff pastry sheet in prepared casserole dish. Allow edges of the puff pastry and the corners to hang over the rim of the casserole dish.
Spoon filling into the casserole dish, spreading it out evenly with the back of a spoon.
Roll out another puff pastry sheet on a floured surface and place it on top of the filling.
Press the edges of the puff pastry sheets together. Roll the overhanging edges together to form a thick rim.
Use a sharp knife to cut a small hole through the puff pastry in the center of the torte.
Whisk together egg yolk and milk.
Brush egg wash over the puff pastry crust.
Bake torte in the oven for 40-45 minutes until crust is a deep golden brown and the filling is hot and bubbly.
Remove torte from the oven and allow it to rest for 15 minutes before cutting into it to serve.
Serve torte with a crisp green salad to round out the meal.