Preheat the oven to 375°F (190°C). Add the chicken, bell pepper, red onion, and corn to a large casserole dish. Sprinkle the fajita seasoning over the top and toss until everything is evenly coated.
Add the rinsed long-grain white rice, then pour in the hot chicken broth. Stir gently and spread everything into an even layer.
Cover the dish VERY tightly with aluminum foil so the steam cannot escape. Bake for 40 to 45 minutes.
Check the rice before adding the cheese. If it is still firm, pour in a small splash of hot chicken broth, cover tightly again, and bake for another 5 to 10 minutes.
Carefully remove the foil and gently fluff the rice with a fork. Sprinkle the mozzarella evenly over the casserole.
Return the dish to the oven and switch to the broiler or top heat. Cook for another 5 minutes, or until the cheese is melted.
Let the casserole rest for about 5 minutes before serving. Garnish with chopped parsley or cilantro if desired, and serve warm with tortilla chips, avocado slices, salsa, or a squeeze of fresh lime.