Rich onion soup made with beef broth and topped with melted cheese is a traditional after-theater late-night snack in French bistros.
Gratinée is a French cooking term referring to a topping of cheese, or bread crumbs and butter, that is browned in the oven or under the broiler until crispy. The crispy surface is considered the best part, and shallow ovenproof dishes made especially for gratins allow for a generous amount of the topping.
For a richer flavor, substitute 1/2 cup dry white wine or sherry for 1/2 cup of the broth.
To save preparation time, use a food processor to slice the onions for this soup.