Cook the potatoes whole (in salted water, you can also blanch the cauliflower in the hot potato water for 3 minutes) and leave them to go cold, then dice into 3-4cm cubes.
Heat the oil in a large pan and cook the cumin and mustard seeds until they begin to pop. Add the cauliflower and spring onions and cook on medium to high heat until the cauliflower takes a deep colour in places. Add 125 ml water, cover the pan and cook for about 10 minutes.
Take off the lid and add the other spices, lemon juice, diced cooked potatoes, salt and pepper to taste. Stir carefully to incorporate everything, and cook for another 5-7 min until the cauliflower is tender and the potatoes are heated through.