Preheat oven to 425 degrees F (220 degrees C).
Melt the butter in a large saucepan over medium heat. Stir in the onions, mushrooms, and garlic; cook until tender, but not browned, about 5 minutes. Stir in the flour, sage, and thyme until blended. Pour in the gravy, water, and milk, stirring to blend. Bring to a boil over medium-high heat; cook 1 to 2 minutes. Stir in the turkey and vegetables, and cook until vegetables are tender, about 5 minutes.
Line a 10 inch pie plate with the bottom crust. Pour in the turkey mixture. Cover with the top crust. Seal and crimp the edges. Pierce top crust in a few places with a fork. Cover the edges of the pie with strips of aluminum foil.
Bake in preheated oven for 25 minutes. Remove foil strips, and bake until crust is golden, about 20 minutes more. Remove from oven, and rest 10 minutes before serving.