Sometimes we just need simple. Good use for leftovers, kid friendly, fast and easy, freezable and delicious. Beat that. (Note: Can sub tuna, drained.) Oh, don't leave out the parmesan. It makes the dish.
Preheat oven 350 degrees and grease a 2 quart baking dish.
In a microwaveable safe bowl, cover and cook the onion and optional bellpepper until tender; about 3-5 minutes.
Combine all remaining ingredients with the onion/bellpepper mixture, except 1/4 cup cheddar cheese. That includes the chicken or turkey, spaghetti, cream of mushroom soup, 1 1/4 cup cheddar cheese, Parmesan, milk, mushrooms, bacon, garlic powder, salt and pepper.
Turn into the prepared baking dish. Sprinkle additional 1/4 cup cheddar on top and bake about 30 minutes or until bubbly.