Kerala Boatman's Crab Curry

Pick over crab meat to remove any bits of shell and cartilage. Chop onion and mince garlic. Peel gingerroot and mince. Wearing protective gloves, break chiles into 1-inch pieces and reserve, along with any seeds. In a 4- to 5-quart kettle heat oil over moderately high heat until hot but not smoking and cook mustard seeds until they begin to pop. Stir in onion, garlic, gingerroot, reserved chiles with seeds, and peppercorns and cook, stirring, until chiles are just browned, 3 to 4 minutes. Stir in other spices until combined well. Add crab and cook mixture, gently stirring occasionally (avoid breaking up crab), about 4 minutes. While crab mixture is cooking, chop cilantro. Stir cilantro into crab mixture with coconut milk and salt and simmer until slightly thickened, 6 to 8 minutes. Spoon curry over rice and garnish with scallion greens.