What gratinating means
Gratinating means browning the top of a dish under a hot grill or in a very hot oven, so it forms a golden, bubbling, crisp crust. The word comes from the French gratiner, and the finished dish is described as gratinée or au gratin. Usually the surface is first covered with something that browns well, such as grated cheese, breadcrumbs, butter, cream or a sauce like béchamel or hollandaise, though sometimes the food itself is browned. Gratinating is often the final stage of a recipe.
Dishes such as cauliflower cheese, macaroni cheese, lasagne, fish pie, French onion soup, stuffed vegetables and seafood in sauce are cooked first and then gratinated to give them colour, flavour and texture. The crisp top contrasts with the soft food underneath, and the browning adds rich, savoury flavours. In restaurant kitchens, gratinating is often done under a salamander, a powerful overhead grill that browns food very quickly. At home, a grill or a hot oven is used, and some cooks use a kitchen blowtorch for small dishes.
The main thing to remember is that browning happens fast, so the dish needs watching closely to stop it burning. Shallow, flameproof dishes work best. You will see the term in French recipes, professional cookery and baked dish recipes.