Prepare the aromatics – Slice the Thai chilis into small slices and remove the garlic peeling. Cut off the coriander roots. Put the chili, coriander, and garlic into a mortar and pestle and grind until you achieve a fine paste-like texture. If you don’t have a mortar and pestle, you can mince each ingredient with a knife.
Make the sauce – Mix the sugar, fish sauce, lime juice, and stock together in a small bowl. Add in the ground aromatics and adjust to taste. The sauce should have more of a soup-like consistency.
Prepare the fish for steaming – Fill a large saucepan or wok about halfway with water. Use a large square of foil to create a circular spiral shape as a makeshift steamer if you don’t have a steamer basket. Place a large plate or serving platter on top of the foil. Add a few bruised lemongrass stalks, then add the filets, salted, into the dish. Turn the stove to medium to high heat to bring the water to a simmer, then turn the heat to low. Cover with a lid for about 3 minutes so that the fish steams.
Dress the fish – After 3 minutes, add the sauce mixture on top of the fish, add the lid back on, and let steam for about 5 more minutes, or until the fish filet becomes opaque.
Garnish and serve – Carefully remove the bowl from the steamer, garnish with cilantro and/or spring onions, fresh lime, and chilies, and serve your delicious Thai steamed fish.