Combine the sauce ingredients in a small bowl; cover and store in the fridge until needed. Place onion, garlic and wine or stock in a large pot and bring to a boil. Add the mussels, cover, and cook until they just open. Drain the mussels well and then cool them to room temperature. Line a platter with lettuce leaves or tarragon sprigs. Remove the top shell from each mussel and discard. Arrange the meat-filled, half shells on the platter. Top each mussel with a small spoonful of the tarragon sauce and serve.