Rub the crabs with 1 teaspoon of the turmeric and 1/2 teaspoon salt; let them marinate for 1 hour. Combine the mustard seed and hot water in a small bowl and let stand for 10 minutes. Use a mortar and pestle to grind the seeds into a coarse paste.
Heat the oil in a wok or kadhai over medium heat. Add the crabs and stir fry until they change colour, about 4 minutes. Remove the crabs from the oil and set aside.
Add the sliced onions to the wok and cook and stir over medium heat until the onions are translucent, about 5 minutes. Raise the heat to high, add the potatoes and cook, stirring constantly, for about 2 minutes. Add the cloves, cinnamon stick, cardamom pods and peppercorns and stir for thirty seconds.
Stir in the tomatoes, ginger paste and garlic paste. Halve three of the chillies and add them to the wok. Cook and stir for an additional minute or two over high heat. Reduce the heat to medium; add the remaining 1 teaspoon turmeric, the cayenne pepper and the mustard paste and stir to combine. Add the crabs to the wok and pour in just enough water to cover the vegetables. Bring the water to the boil and stir in the sugar and salt to taste.
Cover the wok, reduce the heat and simmer until the potatoes are tender and the water is reduced by half, about 10 minutes. Remove the lid, stir and simmer until the gravy is thickened, about 5 minutes more.
Squeeze the lemon wedge over the finished dish. Garnish with chopped coriander and sliced green chillies and serve hot with rice.
Mustard oil can be purchased at Indian/South Asian speciality stores.