Cook cabbage and onion in boiling, salted water until tender. Drain well. Melt butter in a saucepan. Stir in flour until blended. Slowly stir in milk, whisking over medium heat until thick and smooth. Add cheese and stir until melted. Combine cheese sauce with cabbage mixture. Pour into a casserold dish. Top with bread crumbs. Bake at 160ºC or 20 minutes, until lightly browned and bubbly.