Almost any spring and summer vegetable, from asparagus to zucchini, can be successfully and deliciously grilled. Here, however, is a particularly pleasing medley of eggplant, peppers, zucchini, and onion which, when brushed with good olive oil, then drizzled with balsamic vinegar and sprinkled with fresh herbs, become a sort of grilled ratatouille. It is hearty enough to be a main course, but also a wonderful side dish to simple grilled meats, poultry, and seafood.