This gratin sides well with Chicken Tonnato.
Heat oven to 425 degrees. Brush a 9-inch Pyrex pie plate or gratin dish with oil. Trim stem ends of leeks; cut into 1/4-inch-thick diagonal rounds. Place in a bowl of cold water for 5 minutes to rid them of dirt and sand. Pat dry. In a medium bowl, gently toss leeks, zucchini, squash, 2 tablespoons olive oil, and salt and pepper to taste. Arrange vegetable slices in bottom of pie plate, slightly overlapping them.
In a small bowl, combine bread crumbs, Parmesan, garlic, thyme, and salt and pepper to taste. Mix in the remaining 1 tablespoon oil until mixture is crumbly. Sprinkle the crumb mixture over vegetables, and bake until the bread crumbs are golden brown and vegetables are tender, about 25 minutes.