Heat your oven to 220°C/Fan 200°C/Gas 7. Unwrap the beef. Pat dry. Set aside to let the meat come up to room temperature.
Peel and thickly slice the red onions. Place in the bottom of a small or medium roasting tin.
Peel and grate or crush the garlic. Mix it with 1 tsp of the harissa (it’s spicy, so use less if you prefer a milder dish), a little salt and pepper and ½ tbsp olive oil. Rub it all over the beef joint. Pop the beef into the roasting tin on top of the onions.
Cover the cut sides of the beef with squares of foil to keep them moist, allowing just the fat to be directly exposed to the heat. Pop in the centre of the oven, middle shelf. Turn the heat down to 180°C/Fan 160°C/Gas 4. Roast for 45 mins for rare, 1 hr for medium or 1 hr 15 mins for well done.
See our How to Cook the Perfect Steak video for tips on testing the doneness without cutting into the joint. You want to give it a good, long rest (at least 30 mins, longer if possible) loosely covered with foil before carving. Whip up your sides in the meantime.
When it’s done resting, thinly slice the beef. Arrange on a platter. Reheat the pan juices from the beef and drizzle them over the top. Dust with a little salt and ½ tsp harissa to serve.