Preheat the oven to 350 F. Prepare a pie dish with cooking spray and gently press the defrosted crust into the dish. Prick the crust with a fork a few times and pre-bake it for about 10 minutes (until it's lightly browned). Remove and let it cool.
Add 1 TBSP oil to a large skillet or Dutch oven over medium heat. After 2 minutes, add the chicken breast. Season it with salt and pepper, along with half of the dill and half of the thyme. Stir the chicken until it's cooked on all sides and completely through, about 10 minutes. Remove it from the pan and set aside.
Add half of the chicken stock to the pan and use a spatula to scrape up the brown bits (aka deglaze the pan). Add the onions and potatoes with salt and pepper and cook them for about 5 minutes, stirring occasionally. Stir in the carrots, celery and garlic with the remaining thyme and dill. Cover and cook for 7-10 minutes, stirring occasionally. If the pan looks dry, add a splash of chicken stock or water. The veggies are done when the potatoes are tender.
Stir in corn, peas and cooked chicken to the pan of cooked veggies. Sprinkle in 1/4 cup of flour and stir to coat. Then, 1/2 cup at a time, add the milk and remaining chicken stock. Bring the heat up and keep stirring until the mixture starts to thicken. Taste and adjust seasoning as needed; you may want to add more salt or pepper.
Pour the mixture into the pre-baked pie shell. It might not all fit, but the remaining filling is delicious on its own and it's freezer-friendly. Top with the remaining pie shell. Cut 3 slits on top, working from the center of the dish outward. With a small brush or the back of a spoon, brush whisked egg on top of the pie shell. Bake for 20-25 minutes.
You can eat this right away OR let it cool a bit before dividing it into meal prep containers.