Prep the ingredients by chopping the onion and bell pepper, and mincing the garlic.
Cook the chicken by boiling, baking, or grilling until fully cooked, then shred it.
Boil a large pot of salted water, add spaghetti, and cook until al dente, about 8-10.
In a large mixing bowl, combine cooked spaghetti, shredded chicken, cream of chicken soup, chicken broth, half of the cheeses, sautéed onion, bell pepper, garlic, Italian seasoning, salt, black pepper, and red pepper flakes.
Grease a 3-quart slow cooker and transfer the mixture into it, spreading evenly. Top with remaining cheeses.
Cover and cook on low for 4-6 hours or on high for 2-3 hours until cheese is melted and bubbly.