Scrub and boil the potatoes and allow them to cool. Break the potatoes by hand until no pieces are larger than 10 mm cube.
Heat the oil in a wok over a medium heat. Add the mustard seeds and as soon as they begin to pop, add the whole cumin seeds. Sir for a couple of seconds and then add the chopped onion.
Stir fry until the onion is golden (about 4 minutes) and then add the ginger and chilies. Stir for a few seconds until the ginger changes colour then add the ground coriander seeds, ground cumin seeds, turmeric and chili powder.
Stir once then add the broken potatoes and stir fry for 1 minute.
Add 125 ml of warm water, lower the heat and stir gently for 1 minute. Add the lemon juice, salt and garam masala and cook for a further 1 minute, stirring continuously. Remove from the heat.