In a 12" cast iron skillet over medium heat, cook the bacon until crispy and transfer it to a bowl. Leaving the bacon fat in the pan, cook the mushrooms until the begin to turn golden brown. Also transfer them to the bowl.
Add in the chicken pieces and cook until they are golden brown* on the outside, about 8 minutes. Add in the onions and garlic and cook for another 1-2 minutes.
Melt the butter and stir in the flour until it is completely incorporated. Let cook for 1-2 minutes then add back in the bacon and mushrooms. Stir until evenly distributed. Pour in the chicken stock, heavy cream, and milk and cook at a simmer for 5 minutes or until the sauce is thickened. Season with salt and pepper.
Remove the skillet from the heat and cover with the puff pastry. Cut a hole in the center then brush the top with a beaten egg yolk. Put into the preheated oven and bake for 15-25 minutes or until browned to your liking. Remove and let sit for 5-10 minutes before serving so the sauce sets up.
Notes
*Some of the water from the chicken may need to cook out before they begin to brown. This is okay and won't dry out your chicken.