Peel the potatoes and cut them into 1cm/½in dice then boil in salted water. Heat the vegetable oil in a pan and fry the mustard seeds, turmeric and onions until the onions have softened. Drain the potatoes and add to the pan. Season and continue to cook gently. Add the tomato and simmer for a further minute, then add the chopped coriander.
preheat the grill until hot. Brush the fillets with a little melted butter and season. Grill for 4-5 minutes then serve with the bhaji and slices of lemon.