Make sure you set out your puff pastry dough to thaw completely.
In a cast iron, melt the unsalted butter.
Sauté the carrots, onions, and celery in the butter on medium heat until softened, about 3 minutes.
Season the veggies with the garlic powder, thyme, and salt. Add the all-purpose flour and stir until all of the vegetables are covered.
Pour in the chicken broth and heavy cream. Stir and cook for another few minutes until the sauce thickens up.
Preheat your oven to 400°F. Add in your chopped chicken and frozen green peas to the pot pie mixture and stir well.
Remove the skillet from heat and taste the filling. Add any additional seasonings if you prefer.
Take your thawed puff pastry and cover the top of the pot pie filling. I like to cut my puff pastry dough into squares and overlap them on the top to make a pretty design. Whisk the egg with a bit of water. Brush the egg wash onto the puff pastry.
Bake for 16-18 minutes, until golden brown and bubbly. Enjoy!