Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Bring chicken breasts and water to a boil in a large skillet; cook over medium heat until chicken juices run clear and meat is no longer pink inside, about 10 minutes. Transfer chicken breasts to a large plate, reserving pan juices in skillet, and shred chicken with 2 forks.

Cook onion and red bell pepper in reserved drippings over medium heat until onion is translucent, 5 to 8 minutes. Return shredded chicken to the skillet. Stir black beans, corn, picante sauce, chopped green chiles, chili powder, and cumin into chicken mixture until thoroughly combined. Bring mixture to a simmer and cook until heated through, about 5 minutes. Pour chicken mixture into the prepared baking dish.

Arrange flour tortillas in a single layer over chicken mixture. Stir fiesta soup with diced tomatoes and green chiles in a bowl until thoroughly combined; spread soup mixture over tortillas. Top casserole with Mexican cheese blend.

Bake in the preheated oven until cheese topping has melted and casserole is hot, about 30 minutes. Cool 5 minutes before serving.