Preheat a pan. Finely chop the onions and garlic and sauté in a dash of olive oil. Allow to cool. In a separate pan seal the red mullet from one side for 1 minute than let cool. Add the cooked onions and garlic with the red mullet and squeeze in the juice of half a lemon and orange. Finally add the white wine vinegar and the mussels. Allow to marinate for 30 minutes and serve. For the citrus dressing, reduce the fish stock by half, squeeze in the juice of the remaining half lemon and orange and simmer for 10 minutes. Add in the olive oil and chill before serving.