By the first weeks of September, when the evening air can hint of a chill, the grill has lost some of its appeal. We often turn back to the coziness of cooking in the oven. Yet we still enjoy the individual full-bodied tastes found in late summer vegetables. This recipe cooks tomatoes and zucchini together in the oven without having them stew in their own juices. The onions layered at the bottom of the pan lift the tightly packed tomatoes and zucchini out of their juices. Instead of falling apart, the vegetables keep their shape while gently steaming.