Heat sunflower oil in an ovenproof casserole dish and sear the lamb shanks on all sides. Remove from the pan and set aside.
Deglaze the pan with the lamb stock, scraping up all the browned bits that might be stuck to the bottom. Add the tomato puree, rosemary and season well. Remove from the heat.
Add the quartered onions, sliced carrots and the peeled, separated cloves of garlic to the pan. Return the seared lamb shanks to the pan, cover tightly with a lid (or use aluminium foil).
Bake in the centre of the oven for two hours, removing the foil/lid for the last twenty minutes of cooking so that the meat browns nicely.
Carefully remove the lamb shanks and keep warm while you make the gravy.
Dissolve the cornflour in a little cold water.
Return the casserole pan to a medium-high heat on the hob and when the juices in the pan are bubbling carefully pour in the dissolved cornflour. Stir constantly until the gravy thickens. You might need to add some more hot lamb stock or more dissolved cornflour to get it to your desired consistency.
Serve the lamb shanks and gravy with creamy mashed potatos.
Slow Cooker Instructions
Sear meat and deglaze pan, as above. Place in a slow cooker along with the remaining ingredients (except the cornflour) and cook on low for 7-8 hours.