Step 1 Soak the salt cod in cold water for 12 to 24 hours in a sealable plastic bag, changing the water every 4 to 5 hours if possible.
Step 2 Drain the cod, place it in a heatproof bowl or dish and cover it with boiling water. Soak for 15 minutes. Drain, then remove the skin and shred the flesh into small pieces.
Step 3Heat a heavy, large saucepan over medium heat. Add the olive oil. Add the onions and cook until softened, about 5 minutes. Add the ginger, serrano peppers and coriander; cook 1 minute. Add the greens and salt cod and cook until the greens wilt, about 5 minutes. Add the stock and bring it to a simmer, cooking on low heat for 15 minutes. Add the crab, coconut milk, lime juice and sugar. Taste and adjust seasonings, adding more lime juice, salt or sugar if needed. Serve hot.