Cook the potatoes in boiling salted water until almost tender then drain and set aside.
Pour the oil into a large frying pan and place it over a medium heat. When the oil is hot throw in the mustard and cumin seeds and the chilli flakes. As soon as the mustard seeds begin to pop add the fish. Stir fry until the fish is sealed and browned all over.
Add the potatoes, turmeric, coriander and sugar and stir, then add the peas and stir again.
Add 1 cup water and the salt. Bring to the boil then cover, reduce the heat to low and simmer gently for 10 minutes. Add the tomatoes and simmer, uncovered for 3 minutes.
Scatter over the fresh coriander and serve with steamed rice.