Place onion and next 11 ingredients (through crushed red pepper) in a large slow cooker. Gently stir to mix. Nestle chicken breasts in soup mixture and cover. Cook on low heat for 8 hours.
Remove chicken breasts from slow cooker and shred with two forks. Return chicken to slow cooker. Stir in lime juice and add salt and pepper to taste. Serve with tortilla chips, cilantro, sour cream, shredded cheese and avocado, if desired.