Cook spaghetti according to package directions for 7 minutes. Add broccoli; return to a boil. Cook 2 minutes longer or until spaghetti is tender; drain.
In a large bowl, combine soup and milk. Add the mushrooms, onion, water chestnuts, olives, salt, pepper and 1 cup cheese. Stir in spaghetti and broccoli.
Transfer to a 13x9-in. baking dish coated with cooking spray (dish will be full). Cover and bake at 350° for 40 minutes. Uncover; sprinkle with sunflower kernels and remaining cheese. Bake 5-10 minutes longer or until heated through and cheese is melted.