Cook onion in oil and butter in pot over medium heat, stirring frequently, 30 minutes until softened and caramelized. Stir in flour to coat onions. Cook 1 minute. Slowly stir in broth. Cover; cook over low heat 20 minutes. Add sherry if you wish.
Spoon soup into 6 individual oven-proof bowls. Top each with bread and cheese. Broil briefly to melt cheese.