- In a large stockpot or dutch oven, melt butter over medium heat. Add onions, cover, and cook for 10 minutes. Remove lid and cook for another 40 minutes, stirring frequently until onions are lightly browned.
- Add stock and bouquet garni and bring to a boil. Cover and simmer over low heat for about 40 minutes. Remove bouquet garni. Add cognac or sherry and cook for another 10 minutes uncovered. Season with salt and pepper.
- Preheat oven to 425 degrees. Divide soup into six oven-proof bowls. Place a slice of bread on top of each soup bowl and divide cheese evenly among each serving.
- Bake bowls on a baking sheet in middle of oven for about 10 to 12 minutes or until cheese is bubbling and golden brown. Serve hot.