Preheat oven to 325°. In a large saucepan, combine soup, sour cream, 1 cup cheese and 1-¼ cups onions; heat through over medium heat, stirring until blended, 4-5 minutes. Stir in broccoli. Transfer to a greased 2-qt. baking dish.
Bake, uncovered, until bubbly, 25-30 minutes. Sprinkle with the remaining cheese and onions. Bake until cheese is melted, 10-15 minutes.
