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 black beans, drained Rotel, 2 cups of the shredded cheddar, and the taco seasoning to the bowl; 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.
Spoon the remaining 1 cup shredded cheddar over the top of the casserole, then cover the dish tightly with aluminum foil.
Bake covered for 25 minutes.
Carefully remove the foil and bake an additional 10–15 minutes, until the cheese is melted and the casserole is bubbly and the orzo is tender.
Let the casserole rest 5 minutes before serving.