Peel, halve, and small dice onion. Place in a 4-quart slow cooker insert.
Drain and rinse beans. Add to the insert along with tomatoes, cream of mushroom soup, green chilies, water, salt, and oregano. Stir to combine the soup mixture.
Season chicken with spices on both sides, then add to the insert, nestling the breasts into the soup mixture. Cover and cook until chicken is cooked through, about 8 hours on low.
About 10 minutes before the soup is done, cut tortillas into bite-sized pieces. Place on a plate.
Wash, dry, trim, and cut green onions crosswise into thin pieces. Add to the plate with the tortillas.
Coarsely grate cheese, add to the plate, and set aside.
When the chicken is done, carefully remove to a cutting board. Shred the meat using two forks; use one fork to hold the chicken and the other fork to pull and shred the meat. Return chicken to the insert and stir to combine the soup.
To serve, divide tortillas and soup between bowls; top with cheese, green onions, and sour cream. Enjoy!
