Cook cabbage in boiling salted water until tender. Drain well. In a large skillet, saute onion in 5 T. butter until tender. Add soup and mix well. Add cheese. Heat and stir until melted. Remove from heat. Stir in cabbage, salt and pepper. Transfer to an ungreased 2 qt. Baking dish. In a small skillet, melt remaining butter. Cook and stir crumbs in butter until lightly browned. Sprinkle over casserole. Bake, uncovered, at 350 F. for 20-30 minutes. Or until heated through.