Preheat oven to 375. Prepare a 8" square dish with cooking spray; set aside. In a mixing bowl, combine potatoes, egg whites, and milk. Mix until spreading consistency. Spread half onto bottom and up sides of prepared dish. In a small bowl, combine turkey, gravy, carrots, peas, onions, salt, black pepper, and sage. Mix well. Spread over potato mixture in dish. Top remaining potato mixture around edges of dish. Bake for 30 minutes.