Heat the oil in a large saucepan and add the mustard seeds. When they start to pop add the onions and a little salt to taste. Fry over a medium heat until soft.
Add the ground coriander, turmeric and chilli powder. Mix well then add the tomatoes and 4 tbsps water. Cook for 2 minutes.
Add the peas and the pepper. Cover the pan and cook until the peas are tender- about 5 minutes if frozen and 15 if fresh.
Add the fresh coriander and more pepper to taste. Cook for a further 2 minutes.
Serve garnished with fresh corander. Accompany with some flat bread like chapati or naan.