In a large skillet, heat butter until melted. Stir in flour and cook for 1 minute.
Whisk in chicken broth and stir until sauce is smooth and thickened, about 2 to 3 minutes.
Stir chili powder, oregano, salt and pepper into the sauce.
Add chicken breasts to the pan. Bring to a simmer then reduce heat to low. Cover and cook for about 15 minutes until chicken is cooked through.
Remove chicken from the pan and shred into bite-sized pieces.
Stir sour cream into the sauce and return the chicken to the skillet; add tortilla pieces, black beans and diced tomatoes. Stir until combined; top with shredded cheese.
Cover skillet and cook until bubbly and the cheese is melted, about 5-8 minutes.