Heat broiler. Place fennel, bell peppers, and onion on a large broiler-pan rack. Spray with nonstick vegetable cooking spray. Broil 5" from heat 14 to 16 minutes, stirring once, until lightly browned. Remove vegetables from pan to bowl and toss with 1/4 tsp garlic salt and 1/8 tsp pepper.
Combine oil and mustard. Place fish on pan and brush with mixture. Sprinkle with 1/4 tsp garlic salt and 1/8 tsp pepper. Broil 5" from heat 12 minutes or until fish flakes. Serve with vegetables.