Preheat the oven to 180°C (350°F, gas mark 4). With a sharp knife, trim all the fat from the beef joint. Season with the pepper.
Place the fresh herbs in a food processor with the Dijon mustard and process to chop very finely, scraping down the sides of the bowl occasionally. Transfer to a small bowl. (If using dried herbs, just mix with the mustard.)
Spoon the herb mixture onto the beef and spread it evenly to cover all sides. Put the oil in a large roasting tin and add the beef joint. Roast for 30 minutes.
Arrange the onions, courgettes and cauliflower florets in the tin around the joint. Toss the vegetables to coat them thoroughly with the pan juices.
Roast for a further 1 hour or until a thermometer inserted in the centre of the joint reads 70°C (160°F) for medium. Turn the vegetables occasionally so that they cook evenly.
Remove from the oven. With a slotted spoon, transfer the vegetables to a serving dish and keep them warm. Place the joint on a carving board, cover and leave to rest for 5 minutes. Slice the beef and serve with the vegetables.