Preheat the oven to 350°F (175°C) and spray a 9x13-inch baking dish with nonstick cooking spray.
In a large mixing bowl, whisk together the cream of chicken soup, sour cream, and chicken stock until smooth.
Add the cooked chopped chicken, uncooked orzo, frozen corn, drained and rinsed black beans, drained Rotel, taco seasoning, and 2 cups of the shredded cheddar; stir until evenly combined and season with salt and black pepper to taste.
Pour the mixture into the prepared 9x13-inch baking dish and spread into an even layer. Sprinkle the remaining 1 cup of shredded cheddar over the top and cover the dish with aluminum foil.
Bake covered for 25 minutes. Remove the foil and continue baking for 10–15 minutes more, until the cheese is melted and the casserole is bubbly and the orzo is tender.