My mother's version of the French classic. These mussels can be served on a big platter with cocktails, on a buffet, or plated as a first course at a dinner party. Use plump, sweet, clean mussels and make sure to have a fresh baguette ready to mop up the creamy, briny sauce.
In a large pot, melt the butter and saute the shallots and onions until translucent, but not brown. Add the mussels and the rest of the ingredients, stirring to mix. Bring to a boil, cover and simmer for 5-10 minutes until the mussels are opened. Discard any that don't.
Remove the mussels to a bowl and strain the liquid well, through a strainer lined with cheesecloth. Reserve a cup of the broth.
When cool enough to handle, remove the mussels from their shells. Save a half shell for each mussel and spread them out on a large platter. Place one cooked mussel in each shell half.
Prepare the sauce and generously spoon it over each mussel in its shell, so they're completely covered. Garnish with chopped fresh parsley and serve at room temperature or chilled.
For the sauce
Combine shallots, capers, tarragon and parsley. Stir in mayo and Dijon until well blended.
In a small pan, boil the reserved mussel broth until reduced by half. Whisk the hot liquid into the mayo/dijon/herb mixture.
Add salt, pepper and Tabasco to taste (go easy on the salt - the mussel broth is briny.)
Cool the sauce to room temperature before napping the cooked mussels with it.