Rub garlic and thyme all over the beef.
Put the beef joint in a large roasting tray and rub olive oil into the meat.
Cover and leave to marinate in the fridge for one to two days.
Take the beef out of the fridge about an hour before cooking to bring it to room temperature.
Preheat the oven to 190C (170C fan/gas mark five).
Preheat a dry frying pan until very hot, then sear the beef over high heat until it's coloured on all sides.
Place the beef back in a large, hob-proof roasting tray (with the garlic halves) and roast for about 45 minutes for medium rare.
For medium roast beef, roast for a further 12 minutes; for well done, roast for a total of one hour and five minutes.
Transfer the cooked meat to a warm platter, cover loosely with foil, and let the beef rest for at least 20 minutes.
To make the gravy, place the roasting tray on the hob over low heat, add the onions, and gently cook for 20 minutes, stirring occasionally.
Stir in the flour, whisk in the red wine, then boil until the red wine has reduced by half.
Stir in the hot stock, then cook on medium heat for eight minutes.
Carve the beef thinly, serve on dinner plates, and serve the gravy in a jug.
