Gently fry the onions and celery in the butter. Add the beer, pepper and the mussels. Cover. Cook over high heat until the mussels open. Remove the mussels from their shells and keep in a warm place. Strain the mussel juice, reserving 1 cup in a small saucepan. Dissolve the cornstarch with a little cold water and add to the mussel juice Cook over moderate heat for 3 minutes. Whisk the egg yolks into the crÅme fraöche and then whisk into the sauce, off the heat. Put the sauce back on low heat, and cook, whisking continuously, until it thickens. Taste for seasoning and rectify if necessary. Spoon some of the sauce over the mussels. Serve the rest from a sauceboat.
