For brine, bring ingredients and 3 litres of water to the boil in a saucepan. Set aside to cool, then chill in the refrigerator (45 minutes to 1 hour).
Place lamb in a container, pour in brine to cover, place a plate on top to keep submerged and refrigerate for 3 hours to lightly brine. Remove lamb and pat dry with paper towels.
Preheat oven to 110C. Place lamb in a deep roasting pan with garlic, rosemary and chicken stock. Cover with foil and braise until meat is falling off the bone (6-7 hours).
For tomato and pickled onion salad, place onion in a bowl. Combine vinegars, sugar, ginger, chilli, garlic, spices and 120ml water in a saucepan and bring to the boil. Strain liquid through a fine sieve over onion, reserving pickling liquid for another use. Cover with plastic wrap and set aside until completely cold (30-40 minutes). Drain onion, place in a bowl with remaining ingredients and toss to combine.
Meanwhile, for hummus, blend ingredients in a food processor until a smooth purée forms, adding 1-2 tsp water if necessary. Season to taste.
For mint salsa, process mint, parsley, garlic and grapeseed oil in a food processor until smooth. Transfer to a bowl, stir in vinegar, season to taste.
Serve lamb with hummus, tomato and pickled onion salad, and mint salsa alongside.