1. Prepare fish: Fit a wire rack in the bottom of a large roasting pan (17 1/2 by 12 inches) and add about 1 inch of water (it should come just below the top of the rack). Place pan over two burners on stove and bring to a boil over medium-high heat. Meanwhile, rinse fish well inside and out, scraping off any loose scales, and pat dry. Rest the fish on a shallow platter large enough to hold the fish. Tuck the cilantro, half of the lemongrass, and a third of the ginger inside the cavity. Scatter the remaining lemongrass and ginger along with the lime zest, garlic, and half of the scallion over the top of the fish and around the platter. Squeeze half of 1 lime over the fish and drizzle with fish sauce. (Reserve remaining scallion for garnish.)

2. Steam fish: Set the platter on the rack in the pan and cover the pan tightly with parchment-lined foil. Steam over medium-high heat until the fish is cooked throughout, about 10 minutes per inch of thickness (25 minutes for a 2 1/2-inch-thick fish). Test by inserting a sharp knife into the flesh near the backbone; the flesh should be opaque and offer little resistance.

3. Serve: Remove the foil and lift the platter from the pan, being careful not to spill the juices. Garnish with scallion and cilantro and serve immediately.