Melt butter in heavy large Dutch oven over medium-high heat. Add onions and cook until soft and golden, stirring frequently, about 20 minutes. Add stock and simmer 30 minutes. Add wine and simmer 5 minutes. (Can be prepared 1 day ahead. Cover and refrigerate. Bring to simmer before continuing.)
Preheat broiler. Place bread on broiler pan and toast lightly on both sides. Cover each piece of toast with cheese. Return to broiler and cook until cheese bubbles. Place 1 piece of toast in each of 4 shallow soup bowls. Pour soup over croutons and serve.
