Tomato gratin is a French method of preparing tomatoes that involves forming a crispy crust or topping over a casserole that is layered with sliced or very small tomatoes. The crust on top of the tomatoes usually is made from breadcrumbs that have been tossed with olive oil or garlic and, sometimes, grated hard cheese. The tomatoes are layered on the bottom of a baking dish, the breadcrumbs are sprinkled over the top, and the dish is either placed in a very hot oven until the crumbs toast and the tomatoes cook, or the gratin is placed underneath a hot broiler for a short time to crisp the crust. The basic tomato gratin recipe is sometimes enhanced with the addition of spices, liquids such as white wine in the baking dish, or vegetables such as zucchini, onions, potatoes or eggplant. The completed gratin can be served as a side dish, appetizer or very light main course.
The main ingredient and basis for tomato gratin are the tomatoes that are used. Most recipes call for average to large tomatoes. The tomatoes usually are cut into slices that can be easily arranged in the bottom of a baking dish. Alternately, small grape or cherry tomatoes can be used by cutting each tomato in half or quarters, depending on the exact size. A few recipes peel the tomatoes after parboiling them, although this is a matter of personal taste.
The topping for tomato gratin can be made in several ways, but it most traditionally consists of freshly ground breadcrumbs, some olive oil or melted butter, and a little grated Parmesan cheese. The mixture can be made in a bowl and poured over the tomatoes or, alternately, the oil or butter can be melted in a pan along with garlic and other flavorings, after which the breadcrumbs can be added and toasted a bit before being placed over the tomatoes. The breadcrumbs that are used work the best when made fresh from a day-old loaf of bread, largely because premade crumbs purchased in a can might be too fine for the dish and could burn in a hot oven.
To assemble tomato gratin, the tomato slices usually are layered in a casserole dish, sometimes only one or two layers deep, with spices such as salt and basil between each layer. The tomatoes sometimes are cooked for a short time in the oven before the topping is added to ensure they cook without causing the breadcrumbs to burn. The breadcrumb mixture then is poured over the tomatoes and the entire dish is placed in a hot oven or under a broiler. When the breadcrumbs have crisped and the tomato gratin becomes very aromatic, it can be removed from the oven and allowed to cool briefly before being served.