Heat a heavy skillet on high, and cook the steaks for about 3 minutes per side, depending on the thickness of the steaks and how you like them done. (You can also use boneless lamb sirloin. Buy 2 per person and cook for slightly less time).
Mix oil and Worcestershire sauce, then smear over the steaks.
Wrap the cooked lamb in aluminum foil and place on a warm plate while you make the sauce. With the pan on low heat, add butter and let it melt. Finely chop the needles from the rosemary sprig and add them to the pan. (You can substitute oil for the butter to make the dish healthier, although you may lose a bit of the gravy's rich flavor)
Peel and crush or mince the garlic clove into the pan, then pour in the port, letting it sizzle, and reduce slightly. Unwrap the steaks and add any juices from the foil to the sauce. Then put the steaks onto plates and cover with the sauce.
For a side dish, Nigella suggests baby potatoes, orzo, fresh bread, or freshly steamed green beans.