In a Dutch oven, melt butter over medium heat. Saute onions until tender. Sprinkle sugar over onions. Reduce heat and cook, stirring occasionally, until onions are caramelized, about 20 minutes. Add 3 cups broth; simmer 15 minutes. Add remaining broth, Worcestershire sauce, salt and pepper. Cover and simmer for 30-40 minutes.
Meanwhile, spread both sides of the bread with additional butter; sprinkle with garlic salt or rub with the cut-side of garlic clove. Broil bread until golden brown, then turn and brown other side. Ladle soup into individual ovenproof soup bowls. Float a slice of bread in each bowl and sprinkle with cheese. Broil until cheese is melted and bubbly. Serve immediately.