Preheat your oven to 350°F and grease a 9x13-inch baking dish.
In a large mixing bowl, stir together the chicken, rice, cream of chicken soup, sour cream, mayonnaise, onion, cheddar cheese, Parmesan, garlic powder, salt, and pepper until combined.
Spoon the mixture evenly into the prepared baking dish.
In a small bowl, mix the crushed Ritz crackers with the melted butter. Sprinkle this mixture evenly over the casserole.
Bake uncovered for 30–35 minutes, or until the topping is golden and the casserole is heated through.
Remove from the oven, let it cool slightly, then garnish with parsley before serving.