How to Make It
Preheat the oven to 450° and line a large rimmed baking sheet with parchment paper. Arrange the grape leaves in groups of 4 on a work surface, overlapping them slightly. Mound the tomatoes, three-fourths of the julienned scallions and one-third of the sliced serranos in the center of the grape leaves and season lightly with salt and pepper. Season the salmon fillets with salt and pepper and place on top, then wrap the grape leaves around the fish. Carefully turn the packets seam side down on the prepared baking sheet and roast for 10 to 12 minutes, until the salmon is medium within. Transfer the packets to plates.
Meanwhile, in a small saucepan, melt the butter. Add the minced scallion and cook over moderate heat, stirring, until softened, about 2 minutes. Add the vinegar and bring to a boil. Simmer over moderately high heat until reduced by half, about 3 minutes. Whisk in the cream and bring just to a simmer, then cook over moderately high heat, whisking occasionally, until slightly thickened, about 2 minutes. Stir in the remaining serranos and season the sauce with salt and pepper.
Cut open the salmon packets and drizzle the sauce over the fish. Garnish with the remaining julienned scallions and serve right away.