This is a tart and invigorating dish. It can be done ahead of time, and is dressy enough to be either the first course of an elegant dinner or the main attraction of a brunch. Serve with a crisp white wine.
Scrub the mussels under cold water with a stiff brush and remove the beards. Let the mussels stand for 10 minutes in cold water and discard the mussels that either have opened shells or are very heavy.
Put the onions in a large frying pan with the water, salt, garlic, and pepper. Boil, covered, for 10 minutes. Add the mussels and cook, covered, for 4 minutes, stirring occasionally. Remove the mussels with a slotted spoon and discard the shells. Strain the liquid through a piece of cheesecloth spread over a sieve.
Prepare the sauce with the egg yolk, oil, mustard, and lemon juice (for instructions, see Mayonnaise). Add the parsley, then slowly stir in 2 to 3 tablespoons of the mussel broth. Pour the sauce over the mussels, sprinkle with parsley, and chill.
© 1990 Mireille Johnston estate. All rights reserved.