Lightly grease a 6 to 7-quart crockpot. Add the chicken breasts, onion, red bell pepper, and garlic.
In a bowl, mix cream of chicken soup, cream of mushroom soup, chicken broth, sour cream, garlic powder, onion powder, and black pepper until smooth. Pour over the chicken.
Cover and cook on low for 4 to 5 hours or on high for 2 ½ to 3 hours, until the chicken is tender and cooked through.
Remove the chicken, shred it with two forks, then return it to the crockpot and stir well.
Cook the spaghetti separately according to package directions until just tender, about 9 to 10 minutes. Drain well.
Stir the cooked spaghetti and cheddar cheese into the crockpot. Cover for 5 to 10 minutes until the cheese melts and everything is creamy.
Sprinkle with parsley and crushed red pepper flakes if using, then serve hot. Store leftovers in the fridge for up to 3 days.
