Preheat your oven to 425 F and prepare a 9-inch pie dish by spraying with baking spray. Set aside.

Heat a large pan over medium-high heat and add the olive oil to the pan.

Once hot, add the carrots, celery, onion, garlic powder, salt, and pepper. Cook for about 3 minutes, or until the carrots begin to soften.

Sprinkle the flour over the vegetables and cook for ~2 minutes.

Slowly pour in the milk and chicken stock, about ¼ cup at a time, stirring constantly.

Bring to a low boil and scrape any brown bits from the bottom of the pan.

Let it bubble for ~3-5 minutes, or until thickened.

Stir in the chicken, peas, corn, and thyme.

Spoon the chicken and vegetable mixture into the prepared pie dish.

Roll the pie dough into a circle large enough to cover your dish. Lay the dough over the top so that it overhangs the sides. Brush the edges of the pie dish with the egg wash, then trim the overhang to ~1/2 inch larger than the edge of the dish. Gently press the dough onto the sides of the dish so that it sticks, then brush all over with the remaining egg wash. With a sharp knife, cut 3 slits in the top of the dough.

Bake for 20-25 minutes, or until hot and bubbly on the inside and the crust is deeply golden.

Let rest for a few minutes. Serve hot.