Melt butter with oil in heavy large pot over medium heat. Add onions, cover and cook until lightly colored, stirring occasionally, about 45 minutes. Add wine and bring to boil, scraping up any browned bits. Cook 5 minutes. Add browned bits. Cook 5 minutes. Add stock and bring to simmer. Simmer uncovered 1-1/2 hours. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Cover and refrigerate. Bring to simmer before continuing.) Preheat broiler. Ladle soup into boilerproof bowls. Top with slices of toasted French bread. Sprinkle with grated Gruyere cheese. Broil until cheese melts. Serve immediately. SOURCE: Bon Appetit Favorite Restaurant Recipes Volume II Shared by Cate Vanicek