In a large cast-iron pan over medium-high, heat Butter (1 tablespoon) and heat pan until butter is melted. Salt and Pepper (to taste) both sides of the Boneless, Skinless Chicken Thigh (1 pound) and add to the hot pan. Cook for 8-10 minutes until browned on both sides and fully cooked. Remove from pan and set aside. When cooled, cut into bite sized pieces.
Preheat oven to 350 degrees F (175 degrees C).
Wipe away all but 1-2 Tablespoons of grease from the same pan you used for your chicken. Over medium-high heat add your Garlic (2 clove) and cook for 1 minute or until aromatic.
Add your Leek (1), Carrot (2), Celery (1 stalk) and Frozen Green Peas (1 cup) and cook until softened, about 5 minutes.
Add your Low-Sodium Chicken Stock (1 cup) and Beer (1 cup). If you are choosing to omit the beer, add an additional cup of chicken stock. Bring your mixture to a boil and then turn down heat to medium-low. Sprinkle in your All-Purpose Flour (3 tablespoon) and stir until incorporated.
Add your Fresh Rosemary (2 sprig) and chopped chicken and cook for 5 minutes or until thickened. Add Parmesan Cheese (to taste) and stir until incorporated. Add Salt and Pepper (to taste).
Take out your Puff Pastry (1 sheet) about 10-15 minutes before you need to use it. Set your puff pastry directly over the pan. Depending on the size of your pan, you may need to cut the pastry to fill any cracks. You can choose to cut the puff pastry to the size of your pan or let it hang over the sides.
Beat the Egg (1) and brush over the top, then bake for 45 minutes or until crust is golden brown and the mixture is bubbling.
Take care when removing the skillet from the oven! Let cool for a few minutes, then serve.