If a large lamb shank instead of two small ones, the flavors of the spices, cider and coconut milk will have a harder time permeating the thick meat. It might be a better idea to have the butcher slice the shank through the bone into three to four pieces. This gives the meat a better chance to absorb the flavors in the cooking liquid.
Rinse the lamb shanks then pat dry with paper towels.
Heat enough cooking oil to reach a depth of at least three inches in a large wok or frying pan.
Carefully slide the lamb shanks into the hot oil. Cook over high heat until browned. If the shanks are not completely submerged in oil, flip them over for even browning.
Scoop out the lamb shanks and set aside.
Pour off the oil from the pan, leaving only about a tablespoonful.
Throw in the curry powder, kaffir lime leaves, shallots, lemongrass, garlic, ginger and star anise. Cook over medium heat, stirring often, until aromatic.
Put the lamb shanks back in the pan.
Pour in the cider. Allow to boil uncovered for a few minutes.
Pour in the coconut milk. Season with fish sauce. Cover the pan tightly and set the heat to low.
Braise the lamb shanks for an hour or longer, flipping them halfway through the cooking, until the meat is tender. Taste the sauce once in a while and add more fish sauce, if needed.
When the lamb shanks are done, pour in the coconut cream to thicken the sauce. Swirl the pan around to blend evenly. When the sauce simmers, turn off the heat.
Taste the sauce one last time and add fish sauce, if necessary.