Preheat oven to 350°F.
On stovetop in a medium skillet or saucepan, sauté mushroom, onion, celery and garlic in olive oil until veggies are soft; remove from heat.
Meanwhile, in a medium mixing bowl,combine ground turkey, egg, garlic powder, Italian seasoning, salt and pepper.
Add sauté veggie mixture to turkey mixture; mix well.
Stuff peppers with turkey/veggie mix and place in an ungreased shallow casserole dish.
In a small bowl, combine undrained tomatoes and tomato sauce; pour over peppers to cover.
Sprinkle with parmesan cheese and Italian seasonings; cover with aluminum foil and bake for 50 minutes.
Uncover, sprinkle with mozzarella and bake for an additional 10 minutes or until cheese is bubbly and browned.