Ingredients:

  Amount  Measure       Ingredient -- Preparation Method
--------  ------------  --------------------------------
   3      tablespoons   butter
   2      medium        yellow onions -- peeled
   2      pounds        penn cove mussels
   2      tablespoons   olive oil
   2                    garlic cloves ;peeled -- chopped
                        salt pepper
     1/3  cup           dry white wine
   1 1/2  tablespoons   balsamic vinegar
     1/3  cup           heavy cream
                        parsley -- chopped

Preparation:

Heat a large frying pan and add the butter and onion. Sauté over medium heat for 5 minutes. Cover and "sweat" the onion down over low heat. Cook until the onion becomes golden brown and caramelized, about 20 minutes. Be sure to stir the onion often. Set aside. Trim the fuzzy beards off the mussels and wash and drain. Heat a 6 to 8 quart pot and add the oil, garlic, and black pepper to taste. Sauté for 15 seconds and add the drained mussels and wine and bring to a boil. Stir the mussels cover, and reduce the heat to a simmer. Steam the mussels open, stirring once, about 5 to 7 minutes. Drain the mussels into a colander, reserving the broth or "nectar". Heat the frying pan with the onion again and strain in the reserved nectar. Add the balsamic vinegar and cream , and simmer a few minutes to reduce and thicken slightly. Add the opened mussels in their shells and toss until all is hot and coated with the onion sauce. Serve immediately with parsley