Preheat the oven to 130C fan-forced.
Heat 1 tbsp of oil in a large oven-safe pot over high heat. Brown the lamb shanks on all sides, then remove and set aside.
Add the remaining oil and sauté the onion, garlic, and celery until softened.
Stir in tomato paste and cook for 1–2 minutes until slightly caramelised. Add apple cider vinegar and scrape the bottom of the pan to deglaze.
Add the carrots, thyme, rosemary, bay leaves, and lemon peel. Sauté for another 2 minutes.
Return the lamb shanks to the pot and pour in bone broth or water - the shanks should be just submerged.
Cover with a lid and place in the oven for 4 hours.
After 4 hours, stir in the rinsed barley and cook for an additional 30 minutes, until the barley is tender but still chewy.
Remove bay leaves and herb stems before serving. Season to taste.