In a saucepan over medium-low heat, melt butter. Add flour and stir until smooth and bubbly. Remove from heat; slowly add ½ cup chicken broth and ½ cup milk. Whisk to keep smooth. Return to heat. Bring sauce to a gentle boil; cook, stirring constantly, until it thickens. Remove from heat. Add enchilada sauce, remaining milk and remaining broth to heated broth mixture. Whisk until smooth. Set aside.

In a slow cooker, combine drained beans, tomatoes, corn, onion, and bell pepper. Place the chicken breasts on top of the mixture. Pour sauce mixture over ingredients in cooker. Cover; cook on low heat for 6 to 8 hours or on high heat for 3 to 4 hours. When the chicken is cooked through, remove chicken and cut or shred into bite-sized pieces. Add chicken back into the soup and mix well. Top with cheese, sour cream, avocado or tortilla chips, as desired.