Into a slow cooker, add the shredded onion, garlic, tomatoes, cumin, salt and pepper. Stir gingerly (so you don’t bang up your slow cooker). Slide the mixture over and add the chicken; use a ladle or big spoon to completely cover the chicken breast with sauce. Cook on high for 4-6 hours.
Turn slow cooker to warm. Remove chicken and let cool enough to work with. Shred the chicken and put back into the slow cooker. Add white beans and corn. Mix and serve with guacamole, sour cream, tortilla chips or simply plain.