When you have almost nothing in the house, you can still make this soup with old onions, leftover bread, and leftover cheese. It’s always a good idea to keep a bottle of champagne in the fridge.
In a large Dutch oven, over moderately high heat, cook the onionsin the butter, stirring from time to time, for about 30 minutes, until nutty brown. Add the garlic and cook another minute. Add the champagne and the water with salt and pepper and simmer another 30 minutes. Serve in warm bowls with the cheese and croutons sprinkled over each bowl. (Serve with the rest of the bottle of Champagne.)