In a large skillet, saute onions in 3 tablespoons butter until tender, about 25 minutes. In a small bowl, combine flour and pepper. Gradually add broth, stirring until smooth. Add sherry. Stir into onion mixture. Bring to a boil; cook and stir for 2 minutes or until thickened.
Transfer to a greased 1-1/2-qt. baking dish. Melt remaining butter and toss with bread cubes; sprinkle over the onion mixture. Sprinkle with cheeses. Broil 6 in. from the heat until cheese is melted and lightly browned.