I used white wine instead of vermouth and fresh thyme instead of fresh tarragon and here it is-also you can double the portions or adjust to your tastes..
White wine mussels with fresh thyme
1 tbs unsalted butter
about half onion, minced
1/4 to 1/2 cup white wine
After you have cleaned, removed beard, discarded any chipped shells, or those not completely shut, cook the mussels in a medium pot along with the butter, onion and white wine which have been previously boiled. Cover the pot and steam them over high heat until they open. It can take from 3 to 6 minutes. You can shake them around a bit, to make sure they are getting equal access to the wine and heat. Once the mussels have swung open (discard any that have not), transfer them to a bowl with a slotted spoon. Turn the heat all the way up, and boil the cooking broth for 1 minute, then ladle this over the mussels. Garnish with some fresh thyme and eat immediately. :)