Lightly moisten the lamb shanks with water. In a large bowl or plastic bag, combine the flour, salt, and pepper and dredge the meat through that mixture.
In a large heavy based frying pan over a medium heat, heat the rice bran oil. Add the lamb shanks and cook on all sides until browned, about 10 minutes.
Transfer to a deep, oven-proof casserole dish.
Add the onions, carrots, celery, garlic, rosemary and thyme to the frying pan and cook for 5 minutes, stirring constantly to absorb the pan juices. Combine the vegetables and pan juices with the lamb.
Add the Guinness, and beef stock, cover, and simmer for 45 minutes.
Add the potatoes, season again if required, cover and cook until the meat is tender, 90 minutes at least.
To serve, place a lamb shank in the middle of a deep dish plate or bowl and spoon broth and vege gravy all around the lamb. Served atop creamy mash. Spinach and steamed carrots always a winner for winter health.