Allow the steak to come to room temperature before cooking. Remove it from the fridge a good hour beforehand, if possible.
Turn on the grill and place a cast iron frying pan underneath. Allow the pan to heat up along with the grill.
Meanwhile, pat the steak dry with kitchen paper. Rub both sides with a drizzle of oil, then sprinkle liberally with salt and pepper.
Using a well-insulated oven glove, remove the frying pan from underneath the grill and place it on the hob over high heat. The pan should be very hot at this point; if you're unsure, flick a few drops of water on the pan's surface - if they immediately sizzle and burn off, the pan is hot and ready.
Place the steak in the pan and cook for 45 seconds. Flip and cook the other side for 30 seconds, then immediately transfer the frying pan to the grill.
For a rare steak, cook under the grill for 1 minute, then flip and grill for 1 minute more. For medium-rare, cook for 2 minutes on both sides.
Remove the frying pan from under the grill. Using tongs, remove the steak from the frying pan and place it on a plate or cutting board, then tent it with foil. Allow it to rest for 10 minutes for the juices to redistribute before slicing against the grain and serving.