Heat gas or charcoal grill. In large bowl, mix oil, peppered seasoned salt and cumin. Add onion; toss to coat.
When grill is heated, with slotted spoon, place onion in grill basket (grill "wok"); reserve oil mixture in bowl to coat squash. Place basket on gas grill over medium heat or on charcoal grill over medium coals. Cook 10 minutes, turning basket occasionally.
Meanwhile, add summer squash to reserved oil mixture in bowl; toss to coat.
Place squash in basket with onion; cook 5 to 7 minutes longer, turning basket occasionally, until vegetables are tender.