Preheat oven to 375. Prepare a 1.5-quart casserole dish with cooking spray. In a mixng bowl, combine mashed pototoes and half cup cheese. Spoon potato mixture into prepared dish. Using back of spoon, spread pototoes across bottom and up sides of dish to form a shell. In another mixing bowl, combine turkey, vegetables, soup, milk, mustard, garlic powder, and black pepper. Pour over potato shell. Bake for 30 minutes. Top with remaining cheese and bake 3 minutes more or until cheese has melted.