Preheat the oven to 200 degrees Celsius / 400 degrees Fahrenheit
Heat the oil in a frying over medium heat.
When the oil is hot and shimmering, put the piece of lamb rump fat side down and gently fry it until the fat is browned and crispy.
Now sear and seal the other surfaces of the lamb by turning it over onto its sides and ends using tongs to quickly brown off the meat.
Place the lamb onto a lined baking tray and put it in the oven to roast for 15-20 minutes until a meat thermometer measures the middle of the lamb rump to be 55 degrees Celsium / 130 degrees Fahrenheit.
Meanwhile, make your compound rosemary butter. Place the butter into a Tupperware container (one that you definitely have a well fitting lid for) and add the salt, pepper and finely chopped rosemary. Mash together with a fork.
Take the lamb out of the oven and put it straight into the Tupperware with the compound herb butter, swishing it around a bit so the butter melts and coats the lamb. Put the lid on, and leave it to rest for 8-10 minutes.
Carve the lamb rump roast and serve with the butter and juices drizzled over the top