Add all the brine ingredients to a large bowl big enough to take the lamb easily. Don't add the lamb just yet. Set the bowl and ingredients on a scale and fill with water.
Make a note of the weight of the water and then calculate 5% of that - water weight x 0.05. Add that amount of salt (so if your water weighed 1,750g, you'd be adding 87.5g salt). Stir briskly until all dissolved.
Add the lamb and cover, and refrigerate for 12 hours.
Preheat the oven to 250°C. Drain the lamb and discard all the brine ingredients. Pat the lamb dry. Add the halved carrots and onion to a baking tray and set the lamb on top. Roast for 15 minutes and then turn it down to 140°C. Roast for 2 - 3 hours or until the meat reaches 65°C for medium rare.
Video
Notes
Don't like calculating percentages? Shout it at your nearest digital assistant ie. Alexa, Google "what's 5% of 1750...?"