If you can, start this off a couple of hours in advance (or longer, if that suits). All you do is first smear the beef with a little olive oil, then crush the peppercorns coarsely with a pestle and mortar.
Tip them into a fine sieve, which will sift out the really hot inner bits and leave you with the fragrant outer bits, and press these all over the surface of the beef.
When you’re ready to cook the beef, pre-heat the oven to gas mark 8, 450°F (230°C).
Then, to prepare the onions, you first need to mix the sugar and balsamic vinegar together in a large bowl, give the mixture a good whisk, and then leave it to one side for about 10 minutes to allow the sugar to dissolve.
Meanwhile, peel the onions, then, leaving the root intact, cut each one into eight sections through the root, so, in half first, then each half into four. Then add the onions and the tablespoon of oil to the sugar-and-vinegar mixture and toss them around so they get a good coating.
After that, spread the onions out on the baking tray, leaving space for the beef in the centre. Then pour the rest of the dressing over them, sprinkle over the thyme leaves and season well with salt.
Now place the beef in the centre of the baking tray, then into the oven on the highest shelf, and cook for 15 minutes; after that, turn the onions over and return the tray to the oven, giving it another 15 minutes, if you like your beef rare.
For medium, remove the onions after the 30 minutes, keep them warm, and give the beef another 10 minutes with the oven switched off.
If you like it well-done, leave it for another 15-20 minutes.
Either way, keep everything warm while you make the sauce. To do this, put a quarter of the onions into a small saucepan over a medium heat, then stir in the flour to just coat the onions and gradually whisk in the red wine, Worcestershire sauce and balsamic vinegar.
Let it just come up to simmering point, then turn the heat down and simmer gently until the sauce has reduced by about a quarter.
To serve, carve the beef, pouring any escaped meat juices into the sauce, and serve garnished with the onions, the sauce poured over.