Preheat oven to 220°C. Combine herbs and garlic in a small bowl. Trim all white tissue off top of tenderloin by running a sharp knife under 1 end of tissue. Hold end of tissue taut while running the knife along it and pulling it away from meat. Trim off all excess fat.
Starting about 10-12cm along from the narrow end, cut a deep pocket along side of meat until three-quarters of the way through. Spread mustard along inside of pocket, then sprinkle with herb mixture. Return tenderloin to original shape, then tuck narrow end under, to create an even thickness. Tie at 2cm intervals with kitchen string.
Coat tenderloin with oil. Season. Heat a large, flameproof roasting dish over a medium-high heat. Cook, turning occasionally, for 10 minutes or until browned all over. Transfer dish to oven and roast for 15-18 minutes for rare (or until 50°C internal temperature on a meat thermometer) or 20-23 minutes for medium-rare (55-60°C). Remove, cover loosely with foil and rest for 20 minutes to allow juices to settle.
Thickly slice beef and arrange on a platter. Pour over rested juices. Garnish with extra oregano. Serve with bearnaise sauce, if desired.