Scale and remove the fish guts. Clean and score 2 diagonal incisions on each side of the fish. Keep aside and prepare the sauce.
Cut the lemongrass into halves and smash it with the back of your knife.
In a bowl, add all of the ingredients together. Stir gently and give it a taste test. Make sure it's sour and spicy with a hint of sweetness.
Line a platter with aluminium foil if preferred. You can also stream on the platter without the foil.
Plate the fish on the platter/foil. Top with 3/4 of the sauce.
Place some of the lemongrass and ginger in the cavity of the fish.
Steam for about 20 minutes for the whole fish or 10 minutes for the fillet.
Once cooked, pour the remaining sauce over the fish and garnish with more cilantro.
Serve with steamed rice.