1Heat butter in skillet. Slice onions, I use a mandolin for uniform slices. Add onions to butter. Cover and caramelize over low heat, about 30 miuutes. Check on onions about every 10 min. and stir. At the end, splash with a bit of vinegar to bring out the flavor. Add broths, red wine (if using), bay leaves and thyme to a large pot. Add onions. Simmer for about 15 minutes and season with black pepper. Be sure and remove bay leaves and discard.
Preheat broiler. Divide soup between ovenproof bowls and top with a slice of bread. Top with cheese and broil until cheese is melted and bubbling, about 3 minutes.