Preheat oven to 350 degrees. Spray a deep 9 x 13 baking dish with non-stick cooking spray and set aside.
Bring a large pot of water and 1/2 TBS salt to a boil. Place in uncooked spaghetti and cook to al dente per directions on back of package. Drain and place back into pot.
In a large bowl place 4 cups cooked, boneless & skinless chopped chicken. Add in cream of soups, sour cream, spinach, 1/2 of the cheese, red peppers, black pepper, and garlic. Mix well.
Pour mixture into the pot with the cooked, drained spaghetti. Stir well to combine.
Place mixture into prepared baking dish and smooth evenly. Sprinkle remaining cheese on top. Sprinkle fried onions on top evenly.
Bake in preheated oven, uncovered, for 40-50 minutes, until cheese has melted and casserole is heated through.
