Combine all soup ingredients, except the chicken and cornstarch, in a slow cooker insert and stir. Gently add the chicken. Cover and cook on low heat for 6 to 8 hours or on high heat for 3 to 4 hours.
Transfer the chicken to a cutting board or plate and shred into bite-sized pieces. Add the chicken back into the soup.
Optional: In a small bowl, combine the cornstarch and water to make a slurry; stir into the soup to thicken slightly.
To serve, top with your choice of garnishes.