Remove head from fish.
Slash fish crosswise 3 times on each side.
Mix lemon juice, oil, salt and gingerroot; rub cavity and outside of fish.
Cover and refrigerate 1 hour.
Place fish on rack over water in steamer or roaster (water should not touch bottom of rack; if necessary, elevate rack by placing on custard cups).
Cover tightly and heat to boiling; reduce heat.
Steam over boiling water until fish flakes easily with fork, about 20 minutes.
(Add boiling water if necessary) Cut green onions into 2-inch pieces; cut pieces into thin strips.
Prepare Ginger Sauce.
Carefully remove skin from fish; place fish on serving platter.
Pour half of the Ginger Sauce over fish; sprinkle with green onions.
Serve with remaining Ginger Sauce.