Cook and stir turkey in a skillet over medium heat until browned, about 5 minutes. Add potatoes, mixed vegetables, cream of mushroom soup, milk, salt, and pepper to turkey and mix well; pour into a 2-quart casserole dish.
Bring water and butter to a boil in a saucepan; stir in stuffing mix. Cover saucepan and remove from heat; let stand until water is absorbed, about 5 minutes. Fluff stuffing with a fork. Spoon stuffing over turkey mixture and cover dish with a lid or aluminum foil.
Bake in the preheated oven until pot pie is bubbling, 20 to 25 minutes. Remove lid and bake for a few minutes more for a browned topping, if desired.