gratin

French gratin comes from gratter (to scratch, to scrape), from Frankish *kratton. Originally, the gratin was the crust that stuck to the bottom of a baking dish — the part that had to be scraped off. It was the cook's perquisite or the dog's dinner, depending on the household. At some point, the crusty residue stopped being an accident and started being the objective. Cooks began deliberately creating that golden, crunchy layer.

The technique involves topping a dish with cheese, breadcrumbs, or cream and baking it until a golden crust forms on the surface. Gratin dauphinois — thinly sliced potatoes baked in cream — is the most famous version, named after the Dauphiné region of southeastern France. The dish appeared in writing in 1788. Every French region has its gratin: gratin savoyard uses cheese and stock instead of cream, gratin of celeriac uses root vegetable, and so on.

Au gratin became an English cooking term by the nineteenth century. 'Au gratin' means 'with the crust' — it is an adverbial phrase, not an adjective, though English treats it as one. 'Au gratin potatoes' is grammatically nonsensical in French but perfectly understood in English. The word migrated from French technique to English product label without preserving its grammar.

The packaged 'au gratin potatoes' sold in American supermarkets — dried potato slices with a cheese-powder packet — bear the same relationship to gratin dauphinois that instant coffee bears to espresso. The name is the same. The experience is not. The scraped crust on a Le Creuset casserole in a Lyon kitchen and the contents of a Kraft box in an Iowa pantry share only a word.