Combine the fish stock, dry sherry and soy in a large skillet and bring to a boil and then reduce to a simmer. Crush the lemon grass with the back of a knife to release its oils and chop the ginger. Add the lemongrass, ginger and all remaining ingredients except the fish to the simmering liquid and continue to simmer gently for 15 minutes.
The skillet should be large enough so that when the fish is placed in a bamboo steamer, the steamer basket nestles over the skillet tightly. Place lettuce leaves on steamer basket and lay whole sea bass on bed of lettuce. After placing the whole fish in the bamboo steamer and the steamer is nestled over the simmering liquid, cover the basket with a bamboo cover, lower the heat to a gentle simmer and steam gently for 8 to 9 minutes. When completely cooked, remove from the heat, place the fish on a bed of the bok choy and mushrooms and strain the broth. Discard the lemon grass, and ladle a little of the broth over the top. Scatter the more chopped scallions freely as a garnish.