Make the pie dough. In a food processor, add flour and salt and pulse several times. Add butter and pulse until size of peas. In a small bowl, mix egg, cider vinegar and water and whisk well. Add mixture to flour and pulse until dough almost forms a ball. Remove and flatten into a disk and wrap with plastic wrap. Refrigerate dough for 30 minutes.

To a large saucepan over medium high heat, add olive oil. Add onion and leek (optional) and sauté until lightly browned, about 4 or 5 minutes. Add garlic and sauté for 30 seconds. Add carrots, celery, potato, salt, pepper and thyme and cook for 3 minutes. Add wine (optional) and cook for 1 minute. Add chicken stock and bring to a boil. Cook for 10 minutes or until the carrots are almost cooked. Add chicken and peas and cook for 5 minutes.

In a small bowl add cornstarch and water and mix well. Slowly pour the cornstarch slurry into chicken mixture, stirring constantly until the sauce has thickened. Add more cornstarch slurry if it needs to be a little thicker. Cook for 3 minutes.

Coat a 2 quart oven safe casserole dish with cooking spray and pour mixture into dish. Set aside to slightly cool.

Preheat oven to 425.

To a well-floured surface, roll dough out 1/2-inch larger than your casserole dish. Gently roll the dough over your rolling pin and carefully set it on top of the casserole dish so the dough is hanging over the sides. Gently press all around the sides to adhere the dough to the casserole dish. With a sharp knife, make several steam vents on top of casserole.

Place casserole in oven and bake for 25-35 minutes or until the top is golden brown and the chicken mixture is bubbling.

Remove and serve.