Preheat oven to 375. Boil potatoes and minced garlic in water until tender and easy to stick with a fork. Meanwhile in another pot boil celery, carrots and peas until carrots and celery are tender and no longer crunchy. Brown turkey, mushrooms and onions in skillet until onions are tender and meat is fully cooked. Drain and mash potatoes with 1 stick of butter and gradually pour in warm milk until fluffy and creamy. Add salt, pepper and garlic powder to taste. Fold in 1/2 cup of cheese and set aside. Stir in packet of stroganoff sauce to meat and 1 cup of sour cream. Add beef broth until it's slightly creamy, but not runny. Drain and add boiled vegetables to mix mixture and stir until fully coated. Spray an 11x7 cooking dish with cooking spray. Layer bottom of dish with meat and vegetable mixture, next layer green beans, then mashed potatoes and lastly sprinkle top of potatoes with 1/4 cup cheese. Bake for 30 minutes in preheated oven.