Cook spaghetti according to package directions. In large pot, saute chopped onion and garlic. Add spaghetti sauce, cream of mushroom soup, and mushrooms. Add salt and pepper to taste. Let mixture simmer for about 15-20 minutes.
While sauce is simmering, in skillet brown chicken breasts sprinkled with salt and pepper in olive oil on both sides. When brown, add water to skillet and chicken bouillon cubes. Put lid on skillet and simmer at medium heat until chicken breasts are done. This just usually takes about 15-20 minutes. When done, let cool in skillet then remove and shred chicken. Add to sauce that has been simmering.
When spaghetti is done, drain then return to pot. Pour sauce over spaghetti. Turn into greased casserole dish, and cover with cheddar cheese.
Bake at 350°F for 10-15 minutes until bubbly in center. Remove from oven and let set 10 minutes. Enjoy.