Heat oil in a large heavy based pot. Add lamb shanks and cook until well browned on all sides. Transfer to a large plate.
Add garlic, onion, leek, carrots, celery and parsnip to the pot, and cook on low heat for 5-10 minutes or until softened.
Return the shanks to the pot and add beef stock, water and barley. Bring to the boil, then reduce heat to low and simmer, covered, stirring occasionally, for 2 hours, until shanks are very tender. Add more water as required.
Remove the shanks from the pot and shred the meat. Return the meat to the pot (discard the bones) and add parsley. Stir to combine.
Allow to cool. Serve or freeze in airtight containers.
Once defrosted, reheat soup over low-medium heat.