Mustard emboldens the delicate flavor of seafood. Here, Dijon mustard highlights a creamy sauce for chilled, poached mussels.
Melt butter in a medium high-sided saucepan over medium heat. Add shallots and garlic, and cook, stirring, until shallots are translucent and garlic is very fragrant but not brown, about 2 minutes. Add the salt and pepper. Add mussels and wine. Cover saucepan, and cook until mussels open and are cooked through, about 8 minutes. Discard any mussels that have not opened.
Remove mussels from saucepan. Transfer cooking liquid to a small bowl, and set aside to cool. When cool, transfer to refrigerator to chill.
Remove mussels from shells, reserving shells for serving, if desired. Place mussels in a small bowl, cover with plastic wrap, and transfer to refrigerator.
Whisk together creme fraiche, mustard, and champagne vinegar. Stir cooking liquid, and add about 4 tablespoons reserved cooking liquid and shallots to the creme fraiche mixture to make a nice thin sauce with just enough body to cling to mussels. Adjust seasoning with salt and pepper and more mustard if desired. Stir in half the lemon zest and chervil.
Gently toss cold mussels in mustard sauce. Spoon a mussel and a large puddle of sauce back into reserved mussel shells. Sprinkle mussels with remaining half of zest and chervil, and serve with any remaining sauce on the side.
Poach mussels in advance so they have time to chill, and assemble just before serving. Sour cream makes a fine substitute for creme fraiche.