Peel and grate the potatoes. Place in a colander for 15 min to drain.
Meanwhile, in a large frying pan, heat the oil over medium heat.
Add the mustard and cumin seeds and stir for 3 minutes. Take care as the mustard seeds will pop.
Add the onion and saute until soft but not coloured ( around 10 min), then add the cumin, coriander, turmeric and salt. Saute for a further 3 minutes stirring. Remove from the pan and set aside.
Squeeze the grated potato with your hands to remove any liquid and tip into a large bowl. Add the egg, flour, fried onion mixture and taste for seasoning. If necessary, add salt and pepper. Mix well.
Add enough extra oil to the frypan to reach a depth of about 1cm (½ inch). When the oil is hot, carefully add a tablespoon of the potato mixture to make round potato cakes.
Flatten slightly and fry on medium heat for a few minutes each side until golden brown. Drain on a rack.
Serve hot, sprinkled with coriander and a wedge of lemon.