What is the main characteristic of a dish prepared au gratin?

A dish prepared au gratin is primarily characterized by being baked with a crusty topping. This is typically achieved by layering ingredients such as vegetables or pasta with a sauce (often a cream-based sauce) and then topping it with cheese or breadcrumbs before baking. The baking process allows the topping to brown and develop a crispy texture, which enhances the dish's flavor and presentation.

While other cooking methods for preparing dishes exist, such as boiling and steaming, they do not align with the defining features of the au gratin technique. Serving cold with sauces also strays far from what is considered an au gratin style, which is associated with heat and the resulting texture that accompanies baking. Therefore, the signature element of this cooking method is undoubtedly the addition of a baked, crusty topping.