Preheat a barbecue or stovetop grill until very hot.
In a small bowl, stir together ¼ cup of the oil, the garlic, parsley, thyme, and salt and pepper to taste. Set aside.
Brush the vegetables on both sides with the remaining ¼ cup olive oil. Grill (in batches on the stovetop) until browned on the first side, about 5 minutes. Turn the slices and cook until browned on the second side and tender, about 5 minutes more.
Arrange the eggplant and zucchini on a platter, overlapping slightly. Separate the onion into rings and scatter them over the eggplant and zucchini. Drizzle with the dressing.