In a 5- to 6-quart slow cooker, combine onions and butter, stirring well. Cover and cook on High for 45 to 90 minutes, stirring twice, until onions are softened.

Add brown sugar, salt, pepper and soy sauce. Place two clean tea towels, each folded in half (so you have four layers), over top of insert. Cover and cook on High for 3 to 4 hours, stirring three or four times to prevent onions from sticking and burning on edges of insert, until onions are very soft and deep brown.

Stir in beef broth, chicken broth and sherry (if using). Leave tea towels off. Cover and cook on High for 1½ to 2 hours, until very hot.

Preheat broiler. Place baguette slices on a baking sheet and toast on both sides. Sprinkle cheese on one side of each slice and broil for 2 to 3 minutes, until cheese is melted and bubbly.

Ladle soup into serving bowls and top with cheesy bread.