Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish.

In a large bowl, combine the cooked rice, cream of mushroom soup, cream of chicken soup, milk, 1.5 cups of the shredded cheddar cheese, sour cream, and chopped onion (if using). Season with salt and pepper to taste. Stir until all ingredients are well combined.

Pour the rice mixture into the prepared baking dish and spread evenly.

In a small bowl, combine the melted butter with the crushed crackers or breadcrumbs (if using) and the remaining 1/2 cup of shredded cheddar cheese. Sprinkle this topping evenly over the rice mixture.

Bake for 20-25 minutes, or until the casserole is hot, bubbly, and the topping is golden brown. Let it rest for a few minutes before serving.