Preheat oven to 350°F (175°C). Cook spaghetti according to package directions until al dente, then drain.
In a large skillet, melt butter over medium heat. Sauté onions until translucent, about 3 minutes.
1.5 pounds boneless skinless chicken breasts
Add chicken breasts and cook until no longer pink, about 6-8 minutes per side. Remove from heat and shred the chicken.
1.5 pounds boneless skinless chicken breasts
In a large bowl, combine shredded chicken, cooked spaghetti, diced tomatoes, cream of mushroom soup, chicken broth, garlic powder, black pepper, and half of the shredded cheddar cheese. Mix well.
1.5 pounds boneless skinless chicken breasts
Pour the mixture into a greased 9x13 inch baking dish. Sprinkle the remaining cheddar cheese on top.
1.5 pounds boneless skinless chicken breasts
Bake uncovered for 25 to 30 minutes, until the cheese is melted and bubbly.
Remove from oven and let cool for 5 minutes before serving.
Notes
This dish can be made ahead and refrigerated before baking. Reheat covered with foil to avoid drying out.