In a blender or food processor grind the tomatoes, green chillies and ginger. Set aside.
Add the oil to a fry pan, add the mustard seeds and fry until they begin to splutter. Add the curry leaves and onion and cover. Once the onion is browning, add the tomato puree, the green peas, chilli powder and salt to taste. Simmer until thick. Add the rice to reheat, then serve with coriander leaves for garnish.