Penn Cove Mussels in Hard Cider

In a large enameled cast-iron casserole, melt the butter over moderate heat. When it has stopped foaming, add the shallots and cook, stirring occasionally, until softened, about 6 minutes. Whisk in the mustard and cider and increase the heat to moderately high. Add the mussels to the casserole, cover and cook until they begin to open, about 5 minutes. Using a slotted spoon, transfer them to a large bowl as they open; discard any that do not. Continue simmering the broth until reduced by a third, about 4 minutes. Stir in the lemon juice and season with salt. Reduce the heat to low and whisk in the crème fraîche. Return the mussels and any accumulated juices to the casserole and add the tarragon. Stir until the mussels are warmed through and coated with the cider cream, about 1 minute. Serve immediately with crusty bread.