Heat the vegetable oil in a medium non-stick pan and fry the onion until golden brown.
Add the chillies, ginger and garlic pastes, salt, turmeric, garam masala and tomatoes 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 three minutes, until they have opened. Remove them with a slotted spoon and set aside. Reserve the water.
Add the coconut, coriander 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 appetising effect. Add the mussels to the spice mixture and stir thoroughly. Serve with basmati rice.