1) Preheat the oven to 250C/Gas 10.
2) Trim the fins from the fish, rinse and pat dry. Set aside.
3) Rub the bottom of a roasting pan with 1 tablespoon of the olive oil. Place half of the parsley, dill, lemon and onion in the centre of the roasting pan. Make sure that this mound of aromatics is high enough to prevent the fish from touching the bottom of the pan. Rub the fish inside and out with 1 tablespoon of the oil. Season the fish inside and out with the salt and pepper and lay on the bed of aromatics. Place the second half of the aromatics on top of the fish and drizzle with the remaining tablespoon of olive oil.
4) Cover the roasting pan tightly with aluminium foil and cook for 30 to 35 minutes or until the fish reaches an internal temperature of 50C on an instant-read thermometer. Remove the fish from the oven and allow to sit for 10 minutes before transferring whole to a platter lined with the additional fresh parsley.