Adjust oven rack to middle position and set oven to 350°F. Spray a 9x13-inch casserole dish with nonstick cooking spray.
Add oil and onion to a large Dutch oven. Set the pan over medium heat and cook, stirring occasionally, until onion is translucent and has softened.
Add minced garlic and cook an additional 30 seconds, stirring often.
Add chicken broth and deglaze the pan.
Add water, salt, pepper, stir and bring to a boil.
Add dried spaghetti (broken into thirds) and reduce heat to medium-low, and cook spaghetti to al dente (according to package directions).
Add basil, tomatoes, cream of mushroom soup, chicken and 1¼ cups cheese.
Bring mixture just to a low boil and remove from the heat.
Pour the chicken spaghetti into the prepared casserole and sprinkle the topping mixture evenly over the top.
Bake at 350°F for 30-40 minutes or until hot throughout and cheese is melted. If the cheese is browning too quickly, tent with sprayed aluminum foil.
Transfer dish to a cooling rack and let rest 5 minutes before serving.
Garnish with fresh chopped basil and serve with fresh green salad and toasty bread.