Mix together the oil and Worcestershire sauce in a bowl, then smear this over the steaks, either with a pastry brush or by dipping the steaks directly into the bowl.
Heat a heavy-based frying pan over medium-high heat and cook the steaks for about 3 minutes per side, depending on the thickness of the steak and how you like it done. If you prefer to use boneless lamb sirloin or noisettes, I'd advise 2 per person, and less time for their cooking.
Wrap the steaks in aluminum foil to rest on a warm plate or in a warm place, while you make the sauce. With the pan still on the heat, but turned down low, add the butter and let it melt, then finely chop the needles from the rosemary sprig and add them to the pan.
Peel and crush or mince the garlic clove and add it to the pan, then pour in the port, letting it sizzle and reduce slightly. Unwrap the steaks and add any juices from the foil parcel to the sauce. Arrange the steaks on serving plates and drizzle them with the sauce.