Preheat oven to 425 degrees F. Spray an 8-inch or 9-inch square baking dish with cooking spray.
In a large bowl, whisk together chicken broth and condensed soup until smooth. Add the uncooked spaghetti, uncooked chicken, tomatoes, onion, bell pepper, garlic, and ½ cup of the cheese. Stir (or use your hands) to combine everything together.
2 cups low-sodium chicken broth, 1 can (10.5 ounces) condensed cream of mushroom soup, NOT diluted, 6 ounces uncooked spaghetti, ½ lb. uncooked boneless skinless chicken breasts, 1 can (14.5 ounces) petite-diced tomatoes, drained, ½ cup diced onion, ½ cup diced green bell pepper, 2 teaspoons minced garlic, ¾ cup shredded cheddar cheese
Transfer mixture to the prepared baking dish, using your hands or a wooden spoon to gently press all of the ingredients into an even layer. You might need to break the pasta into smaller pieces in order to get as much of it as possible in the liquid. It doesn't have to be perfect, though! Cover tightly with aluminum foil and bake for 40 minutes, stirring halfway through the baking time.
Uncover; stir again. At this point, you should check the pasta to make sure that it is al dente (firm but just about finished cooking). If it’s still too hard, cover the dish and return to the oven until pasta is al dente. Then, move on to the next step.
Top with remaining ¼ cup of cheese.
¾ cup shredded cheddar cheese
Bake, uncovered, for 5-10 more minutes, or until cheese is melted, chicken is cooked through, and pasta is tender. Give the pasta another stir, and then allow the dish to rest for a few minutes before serving. The sauce will continue to thicken a bit as it sits.
Garnish with fresh parsley, if desired. Season with salt and pepper, to taste.
Optional garnish: fresh parsley