Heat the oil in a karhai or deep, heavy-based frying pan over low heat. Add the mustard seeds, cover the pan and wait for the seeds to pop. Add the onion and potato and fry until lightly browned.
Add the turmeric, cumin, coriander and garam masala to the pan and fry for a couple of seconds. Add the tomato and stir until the spices are well mixed. Add the cauliflower florets and stir unt