Season the beef generously with salt and pepper. Heat a little oil in a pan and sear the beef on all sides for 5–10 minutes until browned. Remove from the heat and rest for 5 minutes.
Preheat the oven to 180°C.
Mix the oregano, thyme leaves, rosemary leaves, Dijon mustard, olive oil and lemon juice in a small bowl to form a paste.
Place the prepared vegetables, onions and garlic in a roasting tin. Drizzle with olive oil and port, season with salt and pepper, and sprinkle over a little oregano.
Place the beef on top of the vegetables and brush generously all over with the herb and mustard mixture.
Roast for about 60 minutes, or until the beef reaches your preferred doneness (around 60°C for medium or 70°C for well done).
Remove from the oven and cover loosely. Rest the beef for 10 minutes before carving.
Transfer the roasted vegetables to a serving dish.
Use the roasting juices to make a simple gravy if desired.
Slice the beef and serve with the roasted vegetables and your choice of sides.