Preheat the oven to 375°F (190°C). Grease a 9-by-13-inch baking dish.
Whisk the cream of chicken soup, chicken broth, milk, melted butter, garlic powder, paprika, salt, and black pepper in a large bowl.
Stir in the uncooked rice, cooked chicken, and diced onion until evenly combined.
Transfer the mixture to the prepared baking dish. Cover tightly with foil and bake for 45–55 minutes, checking until the rice is tender.
Remove the foil, sprinkle the cheddar cheese over the top, and bake uncovered for another 5–10 minutes until melted and bubbling.
Rest for 5–10 minutes, garnish with parsley if desired, and serve warm.