Wash the cauliflower and cut it into small pieces. Soak the pieces in salted water for 5 minutes. Drain, rinse again with water, and place in a strainer to remove excess water. Wash and dry the potatoes, leave the skins on, and cut them lengthwise into eight pieces each.

On medium heat, fry the potatoes until the outside is golden brown and crispy. Sprinkle with salt and set aside. In a large skillet, add the mustard oil and heat on high until it starts to smoke. Lower the heat to let the smoke subside and the oil cool slightly. Add the mustard seeds, wait 2-3 seconds, then add the cumin seeds, tomato, and curry leaves. Add the chopped ginger and stir.

Increase the heat slightly and roast the mixture, stirring constantly, until oil starts to separate at the edges. Add the cauliflower, sprinkle with salt, and stir again. Cover and cook on low heat, stirring every 3-4 minutes. Remove the lid, add the green chilies, and continue stirring until the cauliflower is just tender but not mushy. Increase the heat, add the potatoes and amchur powder, and stir. Keep stirring, add the lemon juice, cook for one more minute, then remove from heat. Transfer to a bowl and serve.