Grill the peppers, skin sides down, for about 12 minutes, or until charred. Grill the eggplant, zucchini, yellow squash and onion, turning once or twice, until tender: about 5 minutes for the zucchini and squash, about 6 minutes for the onion and 8 to 10 minutes for the eggplant. Grill the tomatoes, starting cut sides down and turning once, for about 5 minutes, until slightly softened. Lightly brush the bread with olive oil and grill until toasted. Peel the peppers, then arrange all the grilled vegetables on a platter or large plates and keep warm in a low oven.