Heat the olive oil and butter in a large pan and saute the onion and garlic. Add the mussels and when they start opening, add the rest of the seafood. Add the vinegar too, and cook for 2 minutes before adding the boiled rice. Stir well and start pouring gradually the chicken stock while stirring. When the stock absorbs into the rice and you get the desired consistency, season, add the chopped tarragon and fold in the grated parmesan. Serve the Seafood Risotto as shown in the picture.