Season the steak generously with salt and pepper.
Heat a glug of olive oil in the pan until it is hot. You want thesteak to sizzle as soon as it hits the pan. If it doesn't, that means you're just boiling the meat in oil and too much oil will penetrate the meat, making it greasy. If it doesn't sizzle, just take the steak out and wait a bit longer.
Seal the steak on one side in the hot oil. Turn the steak over and add the garlic cloves around it to perfume the meat.
Lay the rosemary sprigs on the steak and add the butter to the pan. Let the butter get nice and hot, then with a metal spoon start spooning up the butter and meat juices and basting the top side of the steak(pouring it over the rosemary to allow the flavour to permeate the meat). Tilt the pan slightly to make it easier for you to spoon up the oil. Careful as you do this, mind. Keep basting.
Remove the rosemary temporarily with tongs. Turn the steak over, put the rosemary back on and resume basting. The oil should sizzle in a satisfying manner when you pour it on the meat. Numnumnum.
Continue for as long as you like your meat done. I'm a medium / medium-rare type of guy and like it pink. Remove the steak from the pan and let it sit for a few minutes before slicing and serving.