You can purchase a loin of beef from any local butcher or supermarket.

set oven to 220 Degrees Celcius, and heat up a tray with several glugs of olive oil in it. 

Meanwhile, season the beef with rock salt and pepper, about tsp of pepper and a tsp of salt on each side.

Place the beef skin/fat side down and seal all sides untill a nice golden brown. (Basically slightly cook on each side, it will be going in the oven anyway).

Immediately put on the middle shelf in your oven, leave in there for 20 minutes and nock the temperature down to 200 degrees celcius.

leave for a further 20 minutes and check with the temperature probe. if its 50 degrees or over its done, if not leave it in there until it is, as long it is over 50 its done. It should be slightly pink in the middle.  But dont worry, it should be.

Leave to rest for 30-60 minutes rapped in tinfoil. Slice into desired portions and serve immediately. (if it need s warming at all, just put it on a plate and put in the oven on full, for literally 5-6 minutes, also checking it to make sure the plate is ok).