Preheat broiler. Line rack of broiler pan with foil.
Finely chop green onion and stir together with mayonnaise.
Pat fish dry and place skin side down on broiler pan.
Season with salt and pepper.
Spread green onion mixture evenly over fish.
Broil 3 to 4 inches from heat and cook for 8 minutes or until green onions are slightly charred and fish is easily flaked with a fork.