Cut halibut into 12 fingers. Season with salt, add to combined marinade ingredients and marinate for about 10 minutes.
Blanch lettuce leaves in boiling water until just wilted, about 45 seconds. Remove from blanching water and rinse under cold water until thoroughly cooled. Drain well, then carefully spread out each leaf, trim off thick stem parts, place on plate, cover with damp towel and set aside.
Return blanching water to medium-high heat and place steamer over water.
To make fish rolls: Place halibut finger, mushroom slice and sprinkle of green onions near base of lettuce leaf. Fold bottom of lettuce over mixture, bring sides in towards centre and continue folding into neat parcel. Place parcel open edge down on large platter that will fit into steamer.
Place finished platter in steamer, cover and steam for 4 minutes.
Meanwhile, in skillet, bring chicken stock to boil. Add chilies and garlic and cook for about 1 minute. Mix oyster sauce, cornstarch and water and add to boiling stock. Continue cooking for about 1 minute or until sauce is slightly thickened. Keep warm.
When fish rolls are cooked, transfer to warm serving plate. Pour juices into sauce and reheat sauce slightly to thicken. Pour sauce evenly over fish, garnish with cilantro and serve immediately.