Set up a steamer with a couple inches of water and bring it to a boil. Lay the bruised lemongrass and cilantro stems on a heatproof plate that fits inside, and rest the fish fillets on top.
Steam the fish over high heat for 8 to 10 minutes, until the thickest part turns opaque and flakes with gentle pressure. Thinner fillets need closer to 6 minutes.
While the fish steams, make the sauce. Stir together the minced garlic, sliced chilies, lime juice, fish sauce, palm sugar, and water until the sugar dissolves. Taste it: it should be sharply sour, salty, and hot.
Carefully lift the plate out. Pour off most of the steaming liquid, then spoon the sauce all over the fish.
Scatter with cilantro leaves and serve right away with steamed jasmine rice.