How to Make It
In a medium saucepan, heat 3/4 cup of the oil. Add the lemongrass and ginger and cook over low heat until fragrant, about 5 minutes. Add the peanut butter, vinegar, sugar, garlic, water and 1 1/2 tablespoons of the fish sauce and simmer for 10 minutes. Let cool slightly and discard the lemongrass. Transfer the mixture to a blender. Add the Sriracha and 2 tablespoons of the lime juice and puree until smooth. Season with salt.
In a pie plate, combine the remaining 2 tablespoons of lime juice, 2 tablespoons of peanut oil and 1 tablespoon of fish sauce. Add the snapper fillets and scallions and turn the fish to coat.
Fill a large, wide pot with 2 inches of water. Set 3 ramekins in the pot and top with a wire rack. Bring the water to a boil. Set the pie plate on the rack, cover and steam until the fish flakes easily with a fork, 8 to 10 minutes. Carefully remove the pie plate.
Transfer the snapper fillets to plates and garnish with the cilantro. Pour the peanut sauce and the liquid from the pie plate into separate bowls and serve alongside.