Preheat the oven to 350°F (175°C).
In a medium saucepan, bring 2 cups of water to a boil.
Add 1 cup Uncle Ben’s long grain white rice to the boiling water, reduce heat to low, cover, and simmer for 18 minutes or until rice is tender and water is absorbed.
While the rice cooks, steam the broccoli florets until tender-crisp, about 4-5 minutes, then drain well.
In a large mixing bowl, combine the cooked rice, cooked chicken, steamed broccoli, condensed cream of mushroom soup, sour cream, shredded cheddar cheese, chopped onion, milk, unsalted butter, garlic powder, salt, and black pepper. Mix thoroughly until all ingredients are well incorporated.
Transfer the mixture into a greased 9×13 inch baking dish and spread evenly.
Sprinkle the shredded mozzarella cheese evenly over the top of the casserole.
Bake in the preheated oven for 25 minutes or until the casserole is hot and bubbly and the cheese on top is melted and lightly golden.
Remove from oven and let stand for 5 minutes before serving.