In a large saucepan, cover the peas with water and bring to a boil. Simmer over low heat until tender, about 45 minutes.
Add a pinch of salt and 3 cups of water and bring to a boil, uncovered, over high heat. Skim off and discard any foam that rises to the surface. Cook until peas are tender.
Pour 1 cup water into a blender and add the chopped onion and the chopped chilis. Add the shredded coconut, salt, cumin seeds, coriander seeds, cardamom and turmeric. Puree to make a rough paste. Set it aside.
Stir the coconut paste into the cooked black-eyed peas and simmer over medium-low heat, uncovered, stirring occasionally, until the flavors mingle, 5 to 10 minutes.
Heat the oil in a small pan over medium-high heat. Add the mustard seeds, cover the pan, and cook until the seeds have stopped popping, about 30 seconds. Stir mustard seeds, oil and coconut milk into black-eyed pea mixture.
Sprinkle with sliced scallions and cilantro and serve warm.
Recipe courtesy of Moss Cafe's Executive Chef, Abigael Birrell