Preheat the oven to 350ﹾF.
Melt 5 tbsp butter in a seasoned cast iron skillet over low heat. Increase heat to medium and cook the celery and onions, stirring until aromatic and translucent.
Slowly mix in the flour, stirring continuously until all has melted together.
Increase heat to medium-high and whisk in the chicken broth, bringing to a boil. Once boiling, reduce heat to low and add the heavy cream, stirring gently and maintaining a simmer.
Add the sage, rosemary, garlic powder, and black pepper. Simmer for five minutes, stirring occasionally. Do a quick taste test – add a pinch of salt or more pepper to your liking.
Add the 1.5 cups of mixed vegetables, stirring gently to coat completely.
Remove the cast iron from the heat. Add the cooked chicken into the cast iron, stirring to coat.
Transfer the mixture from the cast iron to a large bowl, scraping with a spatula to remove all the contents.
Wipe the cast iron pan clean with a paper towel and cover the bottom with the first pie crust. Add the mixture from the large bowl back into the skillet (on top of the pie crust).
Spread the second pie crust over the top, pressing the two crusts together to form a seal. Gently cut a few slits into the top of the pie.
Mix the egg with a teaspoon of water to make an egg wash and brush it over the top of the pie.
Bake for 40 minutes or until the crust is golden brown. Let rest on the counter before serving. Enjoy!