Cut one clove of garlic into slivers, finely chop the remaining two. Stud each lamb shank with a few slivers of garlic. In a deep flameproof casserole heat the oil and brown the shanks well on all sides. Lift out and set aside. Add the onion to the casserole and sauté until golden brown, four to five minutes. Then stir in the cumin, coriander, chilli, sugar, turmeric and ginger. Continue to cook for another three to four minutes. Stir in the tomatoes, coconut milk and fish sauce. Crush the lemon grass with the blade of a heave knife and add to the sauce. Finally add the lime or lemon juice and season well. Simmer the sauce for 10 minutes.
Set oven Gas mark4 180°C(350°F).
Add the lamb shanks to the sauce. Cover and place in the oven for approx. 2 ½ hours or until the meat should be fork tender and fall from the bone. For the last 10 minutes stir in the chopped spinach or chinese leaves.