Bring a medium saucepan of water to a boil. Add the potatoes and carrots and parboil over high heat for 10 minutes. Drain and set aside. Light a grill or preheat the broiler. In a large bowl, combine the olive oil, lime juice, onion, cumin, salt and pepper. Add the potatoes, carrot and zucchini slices and toss to coat. Let stand for up to 15 minutes. Grill or broil the vegetables for about 3 minutes per side, turning once, until nicely browned and tender. Serve hot.