Summer Squash Gratin

Name: Summer Squash Gratin
Ingredients: 4 tablespoons extra-virgin olive oil, divided; 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; Kosher salt; Pepper; 1 cup finely shredded Gruyère (about 2 ounces); 1 plum tomato, very thinly sliced crosswise; Flaky sea salt and crusty bread, for serving