WHY THIS RECIPE WORKS:
We solve the problem of watery tomato gratin by salting the tomatoes for thirty minutes, followed by a spin in the salad spinner to remove excess moisture as well as some of the seeds. Add sweet, soft onions and a crispy, cheesy topping and this dish comes together easily and takes just 25 minutes in the oven. We recommend slicing and salting the tomatoes first, in order to give them time to expel excess moisture, before you begin preparing the other ingredients.
You can find panko, or Japanese-style bread crumbs, in the international aisle of most supermarkets.
3pounds plum tomatoes, cored and sliced into 1/4-inch-thick rings
Salt and pepper
1cup panko bread crumbs
2ounces Parmesan cheese, grated (1 cup)
1/4cup extra-virgin olive oil
1pound onions, halved and sliced thin
1tablespoon chopped fresh thyme
2 garlic cloves, minced
1. Adjust oven racks to lower-middle and upper-middle positions and heat oven to 450 degrees. Toss tomatoes, 1½ teaspoons salt, and sugar together in bowl until combined; let sit for 30 minutes. Combine panko, Parmesan, 2 tablespoons oil, ½ teaspoon salt, and ¼ teaspoon pepper in second bowl.
2. Heat remaining 2 tablespoons oil in 12-inch skillet over medium-high heat until shimmering. Add onions and cook until softened and translucent, about 7 minutes. Stir in thyme and garlic and cook until fragrant, about 30 seconds. Season with salt and pepper to taste. Spread onion mixture in bottom of 13 by 9-inch baking dish.
3. Transfer tomatoes to salad spinner and spin to remove excess moisture. Arrange tomatoes in even layer over onion mixture and sprinkle with ¼ teaspoon pepper. Transfer baking dish to lower-middle oven rack and bake until tomatoes are tender and starting to bubble, about 15 minutes. Sprinkle evenly with panko mixture, transfer to upper-middle oven rack, and bake until topping is golden brown, about 10 minutes longer. Transfer to wire rack and let cool for 10 minutes. Serve.