Cook mussels in wine, covered, in a 5- to 6-quart heavy pot over moderately high heat until mussels just open wide, checking frequently after 4 minutes and transferring open mussels to a shallow baking pan using a slotted spoon. (Discard any mussels that remain unopened after 8 minutes.) When cool enough to handle, detach meat and discard 1 half shell from each mussel. Reserve meat and remaining shells.
Peel and halve egg, then force it through a medium-mesh sieve into a bowl using back of a spoon.
Whisk in oil, vinegar, onion, capers, mustard, parsley, chervil, chives, salt, and pepper until sauce is combined well.
Add mussels, stirring to coat, then chill mussels in sauce, covered, at least 1 hour.
Arrange half shells on a large platter and spoon a mussel with some sauce into each shell.
• Mussels can be chilled in sauce up to 6 hours. • Mussels on the half shell can be assembled on platter 1 hour before serving and chilled.• Instead of serving on the half shell, you can spoon mussels and some sauce over a bed of romaine (and discard shells).
