Heat oven to 500ºF.
Trim the fins from the fish, rinse the fish, and pat dry. Set aside.
Rub the bottom of a roasting pan with 1 tablespoon of the olive oil. Place 1/3 of the parsley, dill, lemon, and onion in the center 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 third of the aromatics into the cavity of the fish and top with the remaining third. Drizzle with the remaining tablespoon of olive oil.
Cover the roasting pan tightly with aluminum foil and cook until the fish reaches an internal temperature of 120ºF on an instant-read thermometer, 30 to 35 minutes. Remove from the oven and allow to sit for 10 minutes before transferring whole to a platter lined with the additional fresh parsley. Serve immediately.