Step 1

For the buttermilk brine, place salt and 600ml water in a saucepan and bring to the boil, stirring until salt dissolves. Cool, then stir in buttermilk and lemon juice, place in a large container and refrigerate to chill (20-30 minutes). Add the lamb shoulder and refrigerate for 1½ days, turning occasionally. Remove lamb from brine (discard brine), then refrigerate lamb uncovered overnight to dry out.

Step 2

For the harissa, dry-roast seeds and peppercorns until fragrant (10-20 seconds). Crush, then combine with remaining ingredients in a food processor and process until coarsely chopped.

Step 3

For the barbecue, burn wood down to embers. Gently grill lamb on a rack over coals, turning often, topping up embers as necessary and ensuring the lamb's surface temperature doesn't exceed 105°C, until well browned (2-2½ hours).

Step 4

Remove lamb from barbecue and place on a double layer of baking paper with shallots. Brush with harissa and wrap to enclose tightly. Grill, turning occasionally, until lamb and shallots are tender (2½-3 hours; try not to open the paper often). Remove lamb from barbecue, keep it well wrapped, and rest in a warm place for 30 minutes. Reserve the juices when you unwrap the parcel and serve spooned over the lamb (reserve the shallots and remaining juices for the pea salad).