Combine the oil, nigella, if using, cumin, and coriander seeds in a large wok, kadai, or frying pan over medium-high heat. Cook, stirring, until the cumin turns golden brown, 2 to 3 minutes.

Add the ginger and cook, stirring, 1 minute. Add the potatoes and stir well to coat with the spices. Then cover and cook until the potatoes have browned some and have begun to soften and stick to the bottom of the pan, 8 to 10 minutes. Stir three or four times during this time and check the cooking. If the potatoes brown too quickly, turn the heat down. If they aren’t browning at all, raise the heat.

Add the cauliflower, ground cumin, cayenne, and fresh chile and stir to coat the cauliflower with the spices. Then cover and cook, stirring and scraping the bottom of the pan three or four times during cooking, until the potatoes are well browned, about 10 minutes.

Sprinkle the vegetables with salt, cover, and continue cooking until the cauliflower is nicely browned and the vegetables are tender, 5 to 10 more minutes. Stir the vegetables every 3 to 4 minutes and check the cooking; if they begin to stick badly and burn, add a teaspoon of water and scrape with a spatula to loosen the spices and crispy bits from the bottom of the pan. Sprinkle with the chopped cilantro and serve hot.