3 red bell peppers, seeded and halved
3 yellow squash (about 1 pound total), sliced lengthwise into 1/2-inch-thick rectangles
3 zucchini (about 12 ounces total), sliced lengthwise into 1/2-inch-thick rectangles
3 Japanese eggplant (12 ounces total), sliced lengthwise into 1/2-inch-thick rectangles
12 cremini mushrooms
1 bunch (1-pound) asparagus, trimmed
12 green onions, roots cut off
1/4 cup plus 2 tablespoons olive oil
Salt and freshly ground black pepper
3 tablespoons balsamic vinegar
2 garlic cloves, minced
1 teaspoon chopped fresh Italian parsley leaves
1 teaspoon chopped fresh basil leaves
1/2 teaspoon finely chopped fresh rosemary leaves
Place a grill pan over medium-high heat or prepare the barbecue (medium-high heat). Brush the vegetables with 1/4 cup of the oil to coat lightly. Sprinkle the vegetables with salt and pepper. Working in batches, grill the vegetables until tender and lightly charred all over, about 8 to 10 minutes for the bell peppers; 7 minutes for the yellow squash, zucchini, eggplant, and mushrooms; 4 minutes for the asparagus and green onions. Arrange the vegetables on a platter. The key to getting those great grill marks is to not shift the vegetables too frequently once they've been placed on the hot grill.
Meanwhile, whisk the remaining 2 tablespoons of oil, balsamic vinegar, garlic, parsley, basil, and rosemary in a small bowl to blend. Add salt and pepper to taste. Drizzle the herb mixture over the vegetables. Serve the vegetables, warm or at room temperature.