Preheat the oven to 375°F (190°C) and apply a light coating of non-stick spray to a 9x13 inch ceramic or glass baking dish.
In a high-capacity mixing bowl, combine the cooked rice, shredded chicken, black beans, corn, and diced green chiles.
Season the mixture with ground cumin and garlic powder, then incorporate 1.5 cups of the enchilada sauce, folding until the components are evenly hydrated.
Transfer the mixture into the prepared baking dish, utilizing a spatula to compress it into an even horizontal layer.
Distribute the remaining enchilada sauce over the surface of the rice mixture.
Combine the Monterey Jack and sharp cheddar cheeses, then disperse them uniformly over the top of the casserole.
Bake uncovered for 25 to 30 minutes, or until the internal temperature reaches 165°F (74°C) and the cheese has achieved a melted, bubbling state with light browning.
Remove from the oven and allow the dish to rest for 5 minutes to set the structure.
Garnish with chopped cilantro and sliced scallions immediately before service.