Prepare all vegetables. Place vegetables in a roasting pan with the mushroom caps and green onions. Season vegetables generously with salt and pepper, and toss with the olive oil and balsamic vinegar. Place vegetables, cut side down, on the hot grill and cook for a couple minutes on each side, or until tender and nicely marked by the grill.
Remove from grill and toss with an additional 1/4 cup balsamic vinegar. Season with salt and pepper, to taste.