Preheat the oven to 350 degrees. Coat a large baking dish with olive oil cooking spray.
Make the crust by placing the flour in a bowl. Add a teaspoon of salt and mix. Add 5-7 tablespoons of ice water. Cut in the shortening with a pastry blender until the mixture resembles a coarse meal. Gently press mixture into a circle, cover with plastic wrap, and refrigerate until ready to use.
Make the pot pie filling by placing 3 cups of chicken broth along with the carrots and potatoes in a large saucepan over medium-high heat for 8-10 minutes, until fork-tender.
Heat butter in a large skillet over medium heat, add onions, and cook, stirring often, for 3 minutes
Add the mushrooms and cook for an additional 4-5 minutes, stirring often, or until softened. Add the minced garlic, stirring constantly for 60 seconds. Season with thyme, sea salt, and freshly cracked black pepper to taste.
Add the roasted chicken to the skillet of mushrooms and onions, along with the carrots, potatoes, broth, and peas; bring to a boil.
Make a slurry by mixing the cornstarch with the remaining cup of COLD broth and mixing thoroughly.
Once the filling is boiling, stir in the well-mixed cornstarch slurry and stir it well. Taste the filling and re-season if needed. Remove from heat.
Pour filling into the prepared large baking dish.
Remove the dough from the refrigerator and roll it out.
Place dough on top of the filling, pressing to the edge of the dish. Cut 4-5 slits in the top of the crust to allow steam to escape.
Cover the edges of the crust with foil to prevent burning.
Bake the pot pie for 45 minutes or until the crust is golden brown.