3 small leeks (1/2 pound), white and tender green parts thinly sliced into rounds
1/4 cup dry white wine
3 medium zucchini, cut lengthwise into 1/8-inch-thick slices, preferably on a mandoline
3 medium yellow summer squash, cut lengthwise into 1/8-inch-thick slices, preferably on a mandoline
1 cup finely shredded Gruyère (about 2 ounces)
1 plum tomato, very thinly sliced crosswise
Flaky sea salt and crusty bread, for serving
Preheat the oven to 425°. In a large skillet, heat 1 tablespoon of oil over moderately high heat. Add the leeks and cook, stirring occasionally, until softened, about 3 minutes. Add the wine and cook until evaporated, about 2 minutes. Spread in a 9-inch round baking dish
Meanwhile, on 2 large baking sheets, spread the zucchini and yellow squash and brush with the remaining 3 tablespoons of oil; season with salt and pepper. Sprinkle with the cheese and let sit until slightly softened, about 5 minutes.
Tightly roll 1 piece of zucchini and set it on the leeks in the center of the dish. Working outward from that center slice, continue rolling and coiling additional pieces of zucchini and yellow squash until you reach the edge of the baking dish. Season the tomato slices with salt and pepper, then tuck in intervals between the zucchini and squash. Scrape any cheese off of the baking sheets and sprinkle on top.
Bake for 30 minutes, until the zucchini and squash are tender and browned in spots. Sprinkle with sea salt. Let cool slightly, then serve with crusty bread.