Heat a large heavy based lidded casserole (Dutch oven). Add the lamb in small batches and brown the pieces all over, putting aside in a medium sized bowl as you go.
Once the meat is browned and removed, add the tablespoon of fat to the casserole, together with the onion wedges, garlic, celery and carrots and cook, stirring from time to time, on a medium heat for 3-4 minutes until slightly singed. If at any point your pan is looking overly brown and parched, a tablespoon of water will help release the sediment, so quickly scrape it up and incorporate into the vegetables before the moisture has evaporated. This adds extra flavour!
Now pour in the broth, throw in the thyme, cover with a lid and place in the oven for 1 hour.
Remove the casserole from the oven and add the turnips. Replace the lid and cook for a further 30 minutes. Take the stew out of the oven again and add the riced celeriac. Put it back into the oven (again, with the lid on) and cook for a final 16-18 minutes or until the celeriac is slightly softened, with just a little bite.
Discard the now leafless thyme stalks, ladle the stew into large bowls and scatter parsley over the top.