Add a few inches of water to the bottom of your steamer and bring it up to a boil over high heat. Make sure to add enough water so it does not evaporate before the fish has finished cooking.
Warm a serving plate that will hold the fish as well as the broth (about 2 cups of broth)
Remove the outer leaves from the lemongrass, cut it into two-inch sections, and smash the lemongrass to release the aroma. Reserve half of this lemongrass to make the broth.
If your steamer is too small to fit the whole fish, cut it in half. Do not season the fish. The seasoning will be taken care of by the very well seasoned broth.
Place the fish onto one or two plates that will fit inside your steamer. Place the pieces of lemongrass and cilantro under and on top of the fish so that they are in direct contact with the fish. **Here I am using a double-decker steamer but you can also place both halves on the same plate, side by side, if necessary**
Steam the fish over high heat until it is cooked through. This may take anywhere 10 and 15 minutes depending on the fish. Use a fork and try to move some of the meat near the thickest part of the fish. If the meat easily pulls away from the bones in the center of the fish, the fish is done. Check the fillets that are in direct contact with the plate, these can sometimes take longer to cook than the fillets on top.
Place the two halves of the fish on the hot plate. Make sure the broth is finished by the time the fish is done cooking.