From America's Test Kitchen
Layering summer’s best vegetables into a gratin can lead to a memorable side dish—or a soggy mess. Juicy summer vegetables like zucchini and tomatoes can exude a torrent of liquid that washes away flavors and turns the dish squishy and soggy. We wanted a simple, Provençal-style vegetable gratin, where a golden brown, cheesy topping provides a rich contrast to the fresh, bright flavor of the vegetables.
To start, we chose our vegetables. The typical combination of tomatoes, zucchini, and summer squash won out (eggplant was too mushy and bell peppers took on a steamed flavor). To eliminate excess moisture, we baked the casserole uncovered, after salting the vegetables. Salting worked like a charm to both season and dry out the zucchini and summer squash, but proved insufficient to deal with all the tomato juice. While we could remove the watery jelly and seeds from the tomatoes, the jelly was crucial for full tomato flavor. We moved the tomatoes to the top gratin layer, which allowed them to roast and caramelize. The roasting not only eliminated moisture but also added flavor, especially when drizzled with an aromatic garlic-thyme oil. Finally, for more complexity, we added a layer of caramelized onions between the zucchini/squash and tomato layers and sprinkled the dish with Parmesan bread crumbs before baking.
Serves 6-8 as a side or 4 as a light main dish
The success of this recipe depends on good-quality produce. A similarly sized broiler-safe gratin dish can be substituted for the 13- by 9-inch baking dish.