Preheat oven: Set to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
Cook the aromatics: In a skillet, heat olive oil or butter. Sauté onion until soft and fragrant, about 3–4 minutes.
Mix the casserole base: In a large bowl, combine uncooked rice, broth, milk, cream of mushroom soup, sautéed onion, vegetables, shredded chicken, and half the cheddar cheese. Season with salt and pepper.
Assemble: Pour mixture into the prepared baking dish. Smooth the top with a spatula. Sprinkle remaining cheddar cheese evenly across the top.
Optional topping: For a crunchy finish, mix breadcrumbs with Parmesan and melted butter. Sprinkle over the casserole.
Bake: Cover with foil and bake for 35 minutes. Remove foil and continue baking for another 15–20 minutes, until rice is tender, the casserole is bubbling, and the top is golden brown.
Rest & serve: Let sit for 5 minutes before serving. Garnish with fresh parsley for color if desired.