Preheat oven to 350°F. Lightly grease 13×9 baking dish.
In a large bowl combine the spaghetti, cream soup, tomatoes with liquid, chicken, milk (or chicken broth), cheddar cheese and garlic powder. Season to taste with salt and pepper.
Transfer to prepared baking dish and top with Monterrey Jack cheese.
Cover with foil and bake for 20 minutes, or until bubbly and cheese is melted. Garnish with green onion, slice and serve.