Preheat the oven to 450 degrees F (220c/ gas mark 7).
Take your beef out of the refrigerator to bring to room temperature, which could take 1 hour or possibly more.
Put the onion slices into a roasting tin and sit the beef on top of them. Use the onion slices as props to help the rib sit up on its bones in an "L" shape. Smear the oil over the white fat of the rib, and sprinkle with the salt, thyme and cayenne pepper. Cook according to the beef's weight and your taste. I like my beef nice and underdone, so I give it 15 minutes per pound (33 minutes per kg) which means, for a joint this size, a cooking time of about 2 hours unless the beef is straight out of the refrigerator, in which case, add another 20 minutes or so. If you want medium beef, give the joint, from room temperature, 20 minutes per pound (44 minutes per kg), and if you like well-done meat, 30 minutes per pound (66 minutes per kg). As for feeding capacity, this size of joint will certainly look over a big tableful, from 8 with lots of leftovers to 14, without the definite promise of them.
When the beef comes out of the oven, remove to a carving board and allow it to rest in a warm part of the kitchen under a tent of foil for 30 minutes before carving; or just leave, tented in its tin, for the same time. Do not start clearing up the tin, even if you have taken the beef out, however, as you will need some of the pan juices and onions for the gravy.
Port and Stilton Gravy:
In a saucepan over low heat, add 2 tablespoons of the fatty juices from the beef tin. To make a roux, whisk in the flour, and then the port. Keep heating and whisking over a fairly gentle heat, until thick and bubbling. If you want to blend the onions into the stock, do so now. Be sure to avoid adding any blackened onions. Put the onions in a blender with the beef stock and mix together. Or leave the stock just as it is, straight out of the tub. Take the saucepan off the heat, and gradually whisk in the beef stock. Whisk in the blue cheese and red currant jelly. Season, to taste, with salt and pepper. Pour in any juices that remain in the tin or on the cutting board. Transfer to gravy boat and serve with the beef.
Recipes from Feast by Nigella Lawson, published by Chatto and Windus