How to Make It
Heat a large cast-iron skillet. Add the anchos and toast over moderate heat, pressing down with a spatula, until fragrant and blistered, about 20 seconds per side. Transfer to a plate and let cool, then cut the chiles into 1/2-inch pieces.
Melt the butter in a large skillet. Add the onions and garlic, cover and cook over moderate heat, stirring, until the onions are softened, about 5 minutes. Add the stock and cook until reduced by half, 6 to 7 minutes. Add the anchos, cream and tarragon and cook over moderately low heat until the onions and anchos are tender and the cream is thickened, about 10 minutes. Add the salt and lime juice, transfer to a bowl and serve.