In a bowl, mix the finely chopped onion, parsley and garlic. Add the olive oil, vinegar, salt, pimentón and pepper. Place the bowl in the refrigerator.
In a large pot, steam the mussels until they open, around 4 - 5 minutes. Remove immediately. Remove the meat from the mussels, being careful not to break into pieces. Add to the bowl with the other ingredients, gently covering the meat.
Thoroughly clean the interior of 1/2 of the mussel shells. When the mussels are chilled, about 10 minutes, scoop up each one with a little of the other ingredients and place them in their shells.
Serve on a platter sprinkled with parsley and a touch of paprika.