Preheat the Oven: Preheat your oven to 240°C (464°F). This initial high heat helps develop a perfect crust.
Prepare the Beef: Season the sirloin generously with salt and black pepper. Optionally, rub with minced garlic and fresh herbs for extra flavor.
Sear the Beef: Heat a roasting tin on the hob with 2 tbsp of oil. Sear the beef on all sides for 2-3 minutes per side until browned.
Roast the Beef: Transfer the beef to the preheated oven. Roast at 240°C (464°F) for 15 minutes, then reduce temperature to 180°C (356°F) and cook for 20 minutes per 450g.
Rest the Meat: Remove the beef from the oven, loosely cover with foil, and let rest for 15-20 minutes before slicing.
Make the Gravy: While the meat rests, remove excess fat from the roasting tin. Add chopped onion and cook until soft. Deglaze with red wine, scraping up browned bits, then add beef consommé. Simmer for 10-15 minutes. Thicken with cornflour if desired.