Heat the oven to 450ºF. Arrange the eggplant and zucchini slabs on one large baking sheet and the pepper and onion slabs on another. Sprinkle the vegetables with the oil, thyme, salt, and pepper and rub with your hands to coat the veggies evenly (just wash your hands afterward).
Roast the vegetables for 20 minutes. Add the mushrooms to the pepper pan and roast until the vegetables are browned, soft, and slightly shrunk, 15 to 20 minutes more. Remove the eggplant, zucchini, and pepper to a cutting board and slice them into strips.
Meanwhile, place the flatbreads directly on the oven rack and bake them until heated through, 2 to 3 minutes. Spread the hummus over the bubbly side of the warm flatbreads. Scatter on the capers and olives. Arrange the roasted vegetables and lettuce on one half of each flatbread. Fold the other half of each flatbread over to cover the vegetables, and serve.
Tip: If you’re not a fan of roasted garlic hummus, replace it with your favorite flavored hummus or classic hummus.