Boil the pot barley in unsalted water for 25 minutes and drain.
Preheat the oven to 160°C.
Peel the onions and slice them into thick rings.
Cut the celery into short lengths.
Peel the parsnip, carrots, and potatoes and cut into chunks.
Pile vegetables into a pot and tuck in lamb chops, thyme, and bay leaves.
Add the corned beef chunks to the pot.
Season with white pepper and add barley and enough stock to cover.
Bring to a slow boil and skim froth from the surface.
Cover with greaseproof paper and a lid and bake for two hours.
Stir in chopped parsley and season with salt and black pepper.
Leave overnight, skim fat the next day, and reheat slowly to serve.