Thaw puff pastry either overnight in the fridge or over a couple of hours at room temperature.
Preheat oven to 400 degrees.
Heat 2 Tbsps butter in a large deep sided pan. Brown the chicken breasts on all sides over medium high heat. Remove breasts to a platter.
Reduce the heat to medium and melt remaining 2 Tbsps butter in the pan. Add the onions and cook about 5 minutes. Add the mushrooms and cook for another 3-4 minutes. Add the garlic and cook another minutes.
Remove the pan from the heat and stir in the flour so the vegetables are coated and the flour is moistened.
Return the pan to the heat and add the chicken back into the pan.
Add the natural apple cider, chicken stock, brandy, hard cider and pepper. Cover and cook over medium low about 20 minutes until chicken is cooked through.
When the chicken is cooked add 1/4 cup of cream and the salt.
Measure the corn starch into a small dish and stir in the remaining 1/4 cup of cream until corn starch is dissolved.
Add the cornstarch mixture to the chicken dish and stir until sauce thickens.
While the chicken is cooking peel and dice the carrots. Put them in a microwaveable dish with a 1/4 of water. Cook on high until they are just tender. This takes about 10 minutes in my microwave but power levels vary so adjust time as necessary.
Roll out puff pastry to approximate the shape of your casserole.
When chicken is done, remove the breasts and chop into bite-size pieces. Keep the chicken pieces and the sauce separate at this point.
Spray a casserole dish about 10″ X 3″ deep with cooking spray. Spoon the chicken into the bottom of the dish.
Next spread a layer of carrots evenly over the chicken. Then spread the frozen peas in a layer over the carrots. Pour the sauce over the top of the vegetables and let it seep down to the bottom of the dish. (If you have more sauce than needed consider reserving some for Brandied Cider Soup).
Cover the pot pie with the pastry and crimp the edges. Cut a few slits in the pastry to allow it to vent.
Bake 45-50 minutes in the oven. If pastry is getting too dark cover loosely with foil. Just lay the foil on top of the pastry. Do not fold down the edges. You do not want the pastry to steam.
If you want to freeze this for a future meal, allow the pot pie to fully cool. Wrap tightly with foil and freeze. To reheat, cook from frozen. Lay foil loosely over the top of the pastry and warm through. This will take about 60 minutes at 350 degrees.