1. Prepare grill.
2. Coat fillets and onions with cooking spray. Sprinkle fish evenly with pepper and salt. Place fish and onions on grill rack coated with cooking spray; cover and grill fish 4 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Grill onions 3 minutes on each side or until charred and tender.
3. While fish and onions grill, combine tomatoes and avocado in a small bowl.
4. Cut grilled onions into 1-inch pieces. Place grilled fish on a serving plate. Top with tomato mixture; sprinkle with grilled onions. Squeeze 1 lime wedge over each serving.