Preheat oven to 150°C. Heat about 1 tablespoon of oil in a large, flameproof casserole pan on medium. Cook celery, garlic and carrot for 5–7 minutes, until soft but not coloured. Stir in tomato paste, wine and stock and bring to boil. Reduce heat to low and simmer.
Meanwhile, heat remaining oil in a heavy–based frying pan on medium and brown lamb shanks all over. Transfer to casserole pan and turn to coat with sauce. Add lemon rind and thyme and season with pepper. Bake for 1½ hours, until meat is very tender.
Transfer lamb shanks from casserole pan to serving dish. Cover loosely with foil and keep warm. Skim any fat from surface of sauce and place casserole pan on cooktop on high heat. Simmer sauce for 5 minutes, until reduced by half and thickened. Season to taste. Pour over shanks.
To serve, spoon mash onto plates, top with shanks, spoon over sauce and scatter with extra thyme.