1. Heat your oven to 220°C/Fan 200°C/Gas 7. Unwrap the beef. Pat it dry with kitchen paper. Finely chop half the rosemary leaves. If you have a pestle and mortar, crush the chopped rosemary with a pinch of salt and pepper. If not, just mix the rosemary, salt and pepper together in a bowl. Rub the mix all over the beef.
2. Peel and finely slice the onions. Spread them out in a small roasting tin. Make a little bed of the remaining rosemary sprigs in the centre. Place the beef on top. If the joint is tall and thin, rest it with a cut-side down and cover the top cut-side with a square of foil to keep it moist. If it’s rounder or squarish with a good layer of fat on top, rest it fat-side facing up and cover each cut-side of the meat with a little square of foil.
3. Pop in the centre of the oven, middle shelf. Turn the heat down to 180°C/Fan 160°C/Gas 4. Roast for 40 mins for rare, 50 mins for medium and 1 hr for well done. If you’re roasting the beef for 1 hr, then add a splash of water to the tin to help keep the onions soft.
4. See our How to Cook the Perfect Steak video for tips on testing the doneness without cutting into your joint. Once done, remove the meat from the roasting tin and let it rest for 30 mins on a board, loosely covered with foil.
5. While the beef rests, put the roasting tin on the hob over a medium-low heat. If your roasting tin isn’t suitable for using on the hob, tip the onions and any tin juices into a small pan. Pick out the rosemary sprigs and pour in 250ml water.
6. Gently cook for 10-15 mins till the liquid has reduced down and the onions are soft. Once the onions are meltingly sweet, swirl in 1 tbsp balsamic vinegar. Simmer for 2-3 mins, then taste and add a pinch more salt and pepper if needed.
7. Thinly slice the beef. Serve with the warm balsamic onions alongside.
8. Tip: If you want to turn your onions into a rich gravy, add 250ml red wine or beef stock. Let it reduce down. For a thicker gravy, place 2 tbsp flour in a mug. Ladle in 4-6 tbsp of the gravy.Whisk till you have a smooth paste. Gradually add the paste back into the onion gravy. Let it gently bubble away till it’s as thick as you like. Repeat using more flour/stock till you have the perfect consistency. Season to taste.