Heat your oven to 220°C/Fan 200°C/Gas 7. Unwrap the beef and pat it dry. Set aside to let the meat come up to room temperature.
Peel the onions and slice each one into 3 or 4 rounds. Put them in a roasting tin. Strip the leaves from most of the thyme sprigs. Tip 1 tbsp paprika and the thyme leaves into a shallow bowl. Roll the beef in the herby spice mix, then rub the mix in to coat all over.
Place the beef in the roasting tin on top of the onions. Tuck the remaining thyme sprigs underneath. Cut 2 squares of foil to cover the cut sides of the beef to keep them moist, allowing just the fat to be directly exposed to the heat. Dust the exposed fat with a good pinch of salt and pepper.
Slide the tin onto the middle shelf of the oven. Turn the heat down to 180°C/Fan 160°C/Gas 4. Roast for 45 mins for rare, 1 hr for medium and 1 hr 15 mins for well done.
Leave the beef to rest for at least 30 mins before carving. Lift the onions out onto a plate and nestle next to the beef. Cover with foil to keep warm. (See our How to Cook the Perfect Steak video at abelandcole.co.uk/recipes for tips on testing the doneness without cutting into your joint.)
Put the roasting tin with the juices from the beef on the hob. Add in a mug of sherry, white wine or stock. Simmer for 15 mins. Skim or pour off any fat. Thinly slice the beef. Arrange on a platter. Strain
and drizzle the juices over the top of your sliced beef. Season with a little salt and a pinch of any leftover paprika. Serve with the roast onions.