Cover chicken breasts with water and bring to a boil and lower to simmer, cooking completely. When cooked, cut into bite-size pieces.
Saute onion. Add cans of navy beans and package of seasoning and combine completely. Add chicken broth from cooked chicken and cooked chicken pieces - season with salt and pepper to taste. Simmer for 20 minutes. Serve with cheese and sour cream. Slices of warmed tortillas also go nicely with this soup!