Dice the onion and bell pepper. Set aside.
Melt butter in a saucepan over medium-low heat. Stir in flour; keep stirring until smooth and lightly bubbly.
Remove from heat and add the chicken broth and half cup milk, a little at a time, stirring to keep smooth. Return to heat. Bring sauce to a gentle boil; cook, stirring constantly, until it thickens. Remove from heat.
In a large bowl, whisk together the enchilada sauce and chicken broth mixture . Gradually whisk in remaining milk until smooth. Set aside.
In a crockpot, combine drained beans, tomatoes, corn, onion, and bell pepper. Place the chicken on top of the mixture. Pour sauce mixture over ingredients. Cover and cook on low heat for 6 to 8 hours or on high for 3 to 4 hours.
When done, remove chicken and cut or shred into bite-sized pieces. Add chicken back into the soup, mix together. Top with cheese and serve. Instead of using Monterrey Jack cheese, I used a Colby Jack/Monterrey Jack mixture. I also left out the jalapenos (per request of my husband).