In a small skillet, cook onion in melted butter unti the onion is tender.
In a large mixing bowl, stir together the cooked onion, condensed soup, process cheese sauce, evaporated milk, thawed chopped broccoli, and rice. Transfer to a 1-1/2-quart casserole.
Bake, uncovered, in 350 degree F oven 40 minutes or until rice is tender. Let stand at room temperature 10 minutes before serving. Makes 4 to 6 servings.