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 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.