For the buttermilk brine, place the salt and water in a saucepan and bring to a boil, stirring until the salt dissolves.
Cool, then stir in the buttermilk and lemon juice. Pour into a large container and refrigerate to chill (20-30 minutes).
Pierce the lamb with a small, sharp knife and insert a piece of garlic and rosemary into each hole.
Add the lamb leg to the buttermilk brine and refrigerate overnight, turning occasionally.
Remove the lamb from the brine (discard the brine), then refrigerate the lamb overnight, uncovered, to dry out.
For the barbecue, burn wood down to embers.
Gently grill the lamb on a rack over medium coals, turning often, topping up embers as necessary and ensuring the lamb’s surface temperature doesn’t exceed 105°C, until the meat is well browned.
Remove the lamb from the barbecue, keep it well wrapped, and rest in a warm place for 30 minutes.
Reserve the juices when you unwrap the parcel and serve them spooned over the lamb.
Slice the lamb before serving.