Slice onions into 1/2-in. Thick rings; saute in butter until tender. Place in a greased 8-in. Square baking dish. In a bowl, stir broth, cream, flour, salt and pepper until smooth; pour over onions. Sprinkle with cheeses. Bake, uncovered, at 350 degrees for 25-30 minutes or until cheese is golden brown and mixture is bubbly. Yield: 8 servings.