Heat your oven to 220°C/Fan 200°C/Gas 7. Unwrap the beef and pat dry with kitchen paper. Set aside on a board to let the meat come up to room temperature - about 30 mins-1 hr should do.
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 very 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, on the 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.
Take the beef out of the oven and lift the joint onto a board or warm plate. Loosely cover with foil and rest for 30 mins. Spoon the onions and roasting tin juices into a cold bowl and leave to cool, so that fat rises and you cna skim it off.
When the beef has finished resting, thinly slice it. Arrange on a platter. Reheat the tin juices and onions in a small pan and drizzle them over the top. Dust with a little salt and ½ tsp harissa to serve.