From America's Test Kitchen Season 10: Old-Fashioned Fruit Desserts
Gratins can be very humble, where the topping is little more than sweetened bread crumbs. Or they can be a bit more sophisticated, as when they are topped with the foamy Italian custard called zabaglione. Zabaglione is made with just three simple ingredients—egg yolks, sugar, and alcohol—but it requires constant watching so that the mixture doesn’t overcook. It also needs to be whisked just long enough to transform the egg yolks to the ideal thick, creamy texture. We were after a foolproof method for this topping for a gratin that could serve as an elegant finale to a special summer meal.
We chose to make individual gratins—perfect for entertaining—and settled on raspberries, strawberries, blueberries, and blackberries. We tossed the berries with sugar and a pinch of salt to draw out their juices and let the mixture sit while we worked on the custard. To prevent scrambled eggs, we kept the heat low; for the right texture, we didn’t stop whisking when soft peaks formed—instead we waited until the custard became slightly thicker. As for flavor, tasters thought that zabaglione made with the traditional Marsala wine was a bit sweet and cloying on top of the berries. We switched to a crisp, dry Sauvignon Blanc and found that its clean flavor allowed the berries to shine. However, with that change, our zabaglione was almost runny. After trying to thicken it with cornstarch and gelatin (with disappointing results), we turned to whipped cream. After carefully folding a few tablespoons of whipped cream into the cooked and slightly cooled zabaglione base, we spooned it over the berries. Finally, we sprinkled the custard with a mixture of brown and white sugar before broiling for a crackly, caramelized crust.
When making the zabaglione, make sure to cook the egg mixture in a glass bowl over water that is barely simmering. Glass conducts heat more evenly and gently than metal. If the heat is too high, the yolks around the edges of the bowl will start to scramble. Constant whisking is required. Although we prefer to make this recipe with a mixture of raspberries, blackberries, blueberries, and strawberries, you can use 3 cups of just one type of berry. Do not use frozen berries. A broilerproof pie plate or gratin dish can be used in place of the individual gratin dishes. To prevent scorching, pay close attention to the gratins when broiling.