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Prepare to the end of step 4 the day before; chill. Let the beef come up to room temperature for at least 2 hours before browning and roasting it.
Remove the beef from the fridge and allow it to come up to room temperature. This will take about 2 hours. Soak the porcini in 250ml boiling water for 30 minutes. In the meantime, finely chop the bacon and fry in a nonstick frying pan until crisp.
Use a slotted spoon to transfer the bacon to a bowl. Add the shallots to the pan and cook on a low heat until soft but not coloured, then add the sage and garlic and cook for 2-3 minutes, stirring occasionally.
Add the shallot mixture to the bowl. Drain the porcini, reserving the liquid to add to the gravy, and finely chop. Mix with the shallots and bacon. Add the breadcrumbs, season and set aside.
Make a cut into the meat between the bones and the fat, lengthways, using a small, sharp knife. You want to create a 5cm deep pocket. Discard any large pieces of fat. Tie a piece of kitchen string around the joint to hold it together and prevent further tearing. Gently rub the olive oil and salt and pepper all over the joint.
Preheat the oven to 180°C, fan 160°C, gas 4, and put the frying pan back on a high heat. Add the beef to the hot frying pan and sear it all over for about 8 minutes so it has a lovely dark crust on each side.
Remove the joint to a plate or board and, once cool enough to handle, gently pack the stuffing into the pocket, pressing it down and along. Using kitchen string, tie around the beef vertically in between the ribs so that the pocket closes and the stuffing is held in place.
Put the onions in a roasting tin and sit the beef on top. Drain the fat from the frying pan and keep it for cooking roasties. Pour half the beef stock for the gravy into the frying pan and bring to the boil, scraping the bottom to release any stuck bits. Pour this into the tin around the beef. Add the rosemary sprigs and bashed garlic cloves.
Roast for 1 hour 40 minutes to 2 hours in the lower half of the oven, until the internal temperature reads 55°C for medium rare, 60°C for medium or 65°C for well done, using a meat thermometer. Cover the bones and stuffing with foil during cooking if they’re darkening too much.
Remove the beef to a carving board and leave it to rest, covered loosely with foil, for at least 30 minutes before carving. While the beef is resting, put the roasting tin on the hob and bring to a gentle boil. In a small bowl, mix the flour with a couple of splashes of port until it becomes a paste. Whisk this paste slowly into the tin and pour in the rest of the port. Simmer for a few minutes to reduce slightly, stirring often. Pour in the reserved porcini liquid and the rest of the beef stock to give the desired gravy consistency. Simmer for 5 minutes and season to taste.
Strain the gravy through a sieve, making sure you press the onions and garlic well to squeeze out the maximum flavour. Pour into a warmed serving jug and serve alongside the rested and carved beef.