1. Heat gas or charcoal grill. Spray 18x14-inch piece of heavy-duty foil or grill pan with cooking spray.
2. Place onion and chiles on foil or pan.
3. Place on grill over medium heat.
4. Cover grill; cook 5 to 8 minutes or until onion and chiles are crisp-tender and just beginning to brown.
5. Add avocados and tomatoes to the onion and chiles. Cover grill; cook about 5 minutes longer or until avocados and tomatoes are hot, turning once halfway through grilling.
6. Remove vegetables from grill to cutting board. Finely chop chiles and coarsely chop remaining vegetables. In medium bowl, place vegetables, cilantro, lime juice, garlic salt and cumin; mix gently until combined. Stir in the cheese.
7. Serve warm. If desired, serve with tortilla chips.