Dust lamb in flour and seal until nut brown. Add garlic and vegetables and allow to soften.
Pour in the port, bring to the boil and add rosemary and lemon peel. Pour in the stock, cover and cook at 160 degrees centigrade for 2 hours.
Remove lamb to one side and reduce the remaining liquid by half. Pass through a sieve. Prepare mash by creaming celeriac and potatoes together, then adding cream, olive oil, parsley and seasoning.
Serve lamb shank on mash with a drizzle of liquid.
