In a large Dutch oven, combine the chicken, water, salt, pepper, celery, onion, and bay leaf. Bring to a boil, then cover, reduce heat, and simmer for one hour.

Once cool enough to handle, remove the chicken and set aside. Discard the vegetables and bay leaf. Shred the chicken into bite-sized pieces.

Skim any fat off the broth in the Dutch oven, then bring it back to a boil. Add the frozen mixed vegetables and cubed potatoes. Cover and simmer for about 8 minutes until tender.

Remove the veggies and potatoes with a slotted spoon. In the same pot, melt butter over low heat. Stir in flour for about a minute to make a roux.

Gradually add back in 3 cups of chicken broth and milk, stirring until thickened.

Stir in seasonings: 1 1/2 tsp salt, 1 1/4 tsp pepper, and thyme. Add the shredded chicken, cooked vegetables, and sliced hard-boiled eggs. Mix gently.

Preheat your oven to 400°F. Pour the filling into a greased 9x13 baking dish. Roll out the pie crust on a floured surface into a rectangle and drape it over the filling.

Crimp the edges and cut a few slits in the top to let steam escape. Bake for about 20 minutes until the crust is golden brown and filling is bubbling up.

Let cool for about 10 minutes before serving.