1. Soak plank in water for 25 minutes.
2. Preheat grill to medium-high heat.
3. Place poblano, jalapeño, and onion on grill rack coated with cooking spray; grill 10 minutes, turning occasionally. Remove from grill; cool. Coarsely chop poblano and onion; finely chop jalapeño. Combine peppers, onion, tomato, cilantro, 1/4 teaspoon salt, 1/4 teaspoon pepper, avocado, and juice from half of lime.
4. Sprinkle salmon with remaining 1/4 teaspoon salt and remaining 1/4 teaspoon black pepper. Place plank on grill rack; grill 3 minutes or until lightly charred. Turn plank over; place fish on charred side. Cover; grill 8 minutes or until desired degree of doneness. Cut remaining lime half into 4 wedges. Top each fillet with 1/2 cup tomato salsa. Serve with lime wedges.