Preheat your oven to 120°C.
Season the roast generously with salt and pepper. Heat the olive oil in a large frying pan over medium-high heat. Sear the roast on all sides until browned, then transfer to a roasting tray.
In the same frying pan, sauté the chopped onion and minced garlic until they become translucent. Add these to the roasting tray.
Arrange the potatoes, carrots, and parsnips around the roast in the tray.
Pour in enough beef stock to come halfway up the sides of the vegetables.
Place the roasting tray in the oven and roast for approximately 2.5 hours, or until the roast reaches your preferred level of doneness.
Remove the tray from the oven. Transfer the roast and vegetables to a serving platter, covering them with foil to keep warm.
Place the roasting tray on the hob over medium heat. Mix the flour with a little cold water to make a paste, then whisk this into the juices in the tray. Simmer, stirring continuously, until the gravy thickens.
Strain the gravy and serve it alongside the roast and vegetables.
Let the roast rest for 15-20 minutes before carving. Serve the carved roast with the roasted vegetables, drizzled with the gravy. Garnish with fresh herbs, if desired.