The inspiration for this recipe came on a recent visit to France. In a small bistro, I was served an elegant but amazingly simple gratin, just a thin layer of sautéed onions with grated Parmigiano-Reggiano on top, baked in a hot oven to form a crisp, fragile delicacy. When I got home, I decided to replicate it—but with a base of thin bread slices underneath the onions, to make it easier to assemble and serve. To my great delight, the bread became wonderfully crisp in the oven, adding more texture, and at the same time captured the delicious onion juices. The key to wonderful flavor here is slowly cooking the onions in a big skillet—they should be meltingly soft without any browning, and moist without excess liquid. Sweet onions are the best—large Vidalia, Maui, Walla Walla, or any other of the fine varieties now available. A gratinate—the Italian term for a baking dish encrusted with cheese or other crisp topping—fills a big sheet pan. It will serve a large group as an appetizer or a lunch dish, or make a great hors d’oeuvre for a crowd, cut in small pieces. You can bake it ahead for convenience, and serve it at room temperature or briefly warmed in the oven.
A hearty home-baked or home-style white bread, in sandwich-loaf shape, is best here. A day-old or 2-day-old unsliced loaf from the bakery is perfect; cut 1/2-inch-thick slices yourself. Otherwise, a good-quality packaged white bread is satisfactory.
