Lift the lamb shanks out of the casserole and place in an oven-proof dish; keep warm in the oven. Strain the sauce into a saucepan and boil steadily until reduced by about half. Add the arrowroot to thicken the sauce slightly if necessary and heat, stirring, until thickened. Place the lamb on a warmed serving plate and pour on the sauce. Garnish with thyme and serve.