Preheat an oven to 375 degrees F (190 degrees C). Thoroughly grease a 9-inch glass pie plate.
Spread mashed potatoes onto the bottom and up the sides of the greased pie plate. Fill potato crust with the turkey, green beans, and gravy. Smooth stuffing on top of the turkey and gravy to create a top crust.
Brush top of pie with melted butter. Bake pie until stuffing is golden and crispy, about 40 minutes. Let stand 5 minutes before serving.