Preheat oven to 400 degrees F. Coat a large shallow roasting pan with nonstick cooking spray. Add eggplant, zucchini, sweet pepper, and red onion to prepared pan. Drizzle with olive oil; sprinkle with herbes de Provence, salt, and black pepper. Toss to coat. Roast vegetables for 30 minutes, tossing once. Add plum tomatoes to roasting pan. Roast for 15 to 20 minutes more or until vegetables are tender and lightly browned in spots.
Meanwhile, rub toasted bread with the cut sides of the garlic clove. Place two small slices or one large slice of the bread on each of four serving plates. Sprinkle balsamic vinegar over vegetables; toss gently to coat. Spoon warm vegetables on top of bread. If desired, garnish with fresh thyme sprigs.