In a medium saucepan over low heat, combine the coconut milk, lime juice, garlic, lemongrass, serrano, turbinado sugar and fish sauce. Whisk it all together, then bring to a simmer and cook for 5 minutes. Cover and keep warm until ready to serve. Remove the lemongrass before serving.
Fill a large pot with 2 inches of water and bring to a light boil over medium heat. Line a bamboo steamer with parchment paper. Sprinkle the halibut with salt and place in the steamer. Cover the steamer and set it over the pot of boiling water. Steam until the fish is cooked through and tender, 6 to 7 minutes.
Transfer the fillets to shallow bowls and top each serving with about 1/2 cup of the coconut sauce. Garnish with a cilantro sprig and serve.
Cook's Note: Cod or sea bass can be substituted for halibut.