Heat the oil in a heavy frying pan. Add the coriander and cumin seeds and fry until brown. Add the ginger, chillies, turmeric and salt. Add the potatoes and cook, stirring, until the potatoes are tender. If the potatoes start to stick to the pan, add 1/2 cup water.
Add the cauliflower and the rest of the water. Cook until tender and all the flavours are blended. Add the peas and cook a few more minutes. Add the lemon juice, coriander and yoghurt, stir through and serve.