Prepare grill or broiler. Place corn on grill rack or broiler pan coated with cooking spray; grill 15 minutes or until corn is lightly browned, turning every 5 minutes. Cool. Cut kernels from ears of corn to measure 2 1/2 cups. Grill or broil onion slices 2 minutes on each side. Cool. Chop onion slices to equal 3/4 cup.
Combine lime juice and next 5 ingredients (lime juice through garlic) in a large bowl; stir well. Add corn kernels, chopped onion, tomato, and green onions; stir well. Serve at room temperature or chilled with fat-free baked tortilla chips.