Au gratin is a French cooking term for a dish finished with a browned, crisp crust. The word gratin comes from the French gratter, meaning to scrape, a nod to the prized crispy layer that forms on top and around the edges of the dish.
That crust is the whole point. Underneath, the vegetables stay soft and creamy in their sauce, while the top bakes golden. Every spoonful gets both textures at once.