Cook rice if needed; 6 cups cooked rice is about 2 cups dry rice.
Add rice, broccoli, cream of mushroom soup, and Cheez Whiz to a 3-quart baking dish.
Stir until evenly combined.
Season with salt and pepper to taste.
Bake uncovered at 350°F for 45 minutes, until set and lightly golden around the edges.
Let sit a few minutes before serving.
Notes
Drain the broccoli thoroughly so the casserole does not get watery. Taste before adding salt because the soup and Cheez Whiz are already salty. Add cooked diced chicken to turn it into a main dish. Leftovers keep refrigerated for up to 4 days and reheat with a splash of water if thickened.