In a food processor, combine the cilantro, garlic and lemon juice and process to a paste. With the machine on, pour in 2 tablespoons of the olive oil. Scrape the cilantro pesto into a small bowl and season with salt and pepper.
In a large skillet, heat the remaining 3 tablespoons of oil. Add half of the shrimp and cook over moderate heat, stirring, until barely cooked through, about 4 minutes. Transfer to a plate and repeat with the remaining shrimp.
Add the scallions and jalapeño to the skillet and cook over moderate heat, stirring, for 1 minute. Add the wine and simmer until reduced by half, about 3 minutes. Add the fish stock and simmer over moderately high heat until reduced by half, about 4 minutes. Return the shrimp to the skillet with any accumulated juices. Stir in the cilantro pesto and bring just to a simmer. Season with salt and pepper and remove from the heat. Stir in the butter and serve.