Special equipment: mortar and pestle
Heat the vegetable oil in a medium nonstick pan and fry the onion until golden brown. Add the tomatoes, chiles, ginger paste, garlic paste, garam masala, turmeric, and salt, to taste, and cook until the tomatoes have softened and start to break down. Meanwhile, cook the mussels in a large pot of boiling water for about 3 minutes, or until they have opened. Remove the mussels with a slotted spoon, and set aside. Reserve the cooking water.
Add the coconut, cilantro, and a splash of the cooking water to the spices and tomatoes, and stir. The spice mixture should be thick enough to coat the mussels and enter the shells.
Remove some of the mussels from their shells and leave others in their shells, for a dramatic and appetizing effect. Add the mussels to the spice mixture and stir thoroughly. Serve with basmati rice.