Simmer the celeriac in salted water along with the garlic and rosemary for approx. 20 mins. until soft, drain and remove the rosemary. Place the celeriac in a blending cup along with all the other ingredients up to and including the salt, puree and keep warm.
Heat the oil in a non-stick frying pan, fry the spring onions for approx. 5 mins., turning occasionally, season.
Combine the mustard with all the other ingredients up to and including the pepper, rub the mixture into the lamb chops. Heat the oil in a non-stick frying pan. Fry the lamb chops in batches for approx. 3 mins. on each side, cover and leave to rest for approx. 5 mins. Serve with the puree and spring onions.