Rinse the fish under cold running water, pat dry, and place on a lightly oiled, heatproof platter that will fit in your steamer or a large roasting pan. Cut 3 deep diagonal gashes along each side of the fish, to allow the flavors to penetrate the meat. Sprinkle the inside cavity and outside of the fish with the salt, squeeze the lemon juice over the fish, drizzle with sesame oil, then stuff the slivered ginger and chopped garlic into the gashes. Scatter the scallions over the fish, and marinate at room temperature for 20 minutes.

Fill the steamer or roasting pan with water to within an inch of the steamer rack, and bring to a boil. Reduce to a simmer, and add the ginger chunks to the water. Place platter with fish on steaming rack, cover, and steam over low heat for 20 to 25 minutes, depending on the thickness of the fish (plan on 10 minutes for each 1inch of thickness at its widest point).

About 5 minutes before the fish is done, heat the peanut oil and the sesame oil in a small saucepan until hot, about 3 to 5 minutes. Remove from the heat and stir in the soy sauce, lemon juice and sugar.

Test fish for doneness by poking with a fork or chopsticks. The fish meat should flake easily at its thickest part. Remove from the steamer, pour the hot oil mixture over the fish, and serve immediately.