Heat oil, mustard seeds, and urad dal. Roast for a few seconds. Add vegetables. Sprinkle salt and turmeric. Cook on high heat for a couple of minutes. Add coriander powder, chili powder, and cumin powder, (and ginger and garlic if you wish) cover and cook on low until done. For making the potatoes more crisp, add rice powder (arshi maav) and stir it in.