Preheat your oven to 350 degrees. Mist a 9x13 pan with cooking spray and set aside. 

Heat the olive oil over medium heat in a small skillet. Add the onion and celery. Cook 2 minutes or until soft.

Stir together the cooked onion, celery, cheese, turkey, stuffing, and green onion in a medium sized bowl. 

Spoon about 1 tablespoon of mashed potatoes and 2 tablespoons of the stuffing mixture to each cooked shell. Add each filled shell to your prepared pan. 

Bake 20 minutes or until heated through. Serve with gravy.