Line a bamboo steamer with parchment so it comes up over the sides. Line the parchment with the onion slices, some ginger slices, and one slice of lemon. Lay your fish on top of these aromatics. Stuff the inside of the fish (inside the belly as well as inside the head) with most of the remaining ginger and lemon, lemongrass, some of the leeks, a few stalks of cilantro, and cilantro root. Top the fish with any ginger and lemon you have left, the rest of the leeks, and the most of the cilantro (saving a few stalks to garnish fresh once the fish is cooked). Top everything with the soy sauce, fish sauce, and sesame oil. I’ve made the amounts above flexible as this really depends on your taste and the size of your fish. Fold sides of parchment over the fish – it doesn’t have to cover it completely – and cover.
While you are arranging your fish, fill a wok with enough water not to touch the steamer when you lay it inside. Cover and bring to a boil, then lower to a fast simmer.
When the fish is ready and the water in the wok is at a fast simmer, carefully place steamer on the wok.
Steam fish for about 20 minutes. You may need more or less time depending on the size of your fish so check for doneness before taking it off the heat. Simply open the steamer (carefully! Steam burns!), and insert a small knife into the fleshiest part of the fish – if it flakes easily it’s done.
Turn of the heat and remove the bamboo steamer from the wok – do this carefully with your hands protected as very hot steam will escape when you lift the steamer from the wok. Place the steamer on a plate and serve directly! Tear the remaining cilantro over the newly cooked fish.