In a large cast-iron or other ovenproof skillet, saute onions in 3 tablespoons butter until tender. Stir in flour and pepper until blended; gradually add broth and sherry. Bring to a boil; cook and stir until thickened, about 2 minutes., In a microwave, melt remaining butter. Add croutons; toss to coat. Spoon over onion mixture. Sprinkle with cheeses. , Broil 3-4 in. from the heat until cheese is melted, 1-2 minutes.