1. Put oven rack in upper third of oven and place a shallow baking pan on the rack. Preheat oven to 475 degrees.
2. Toss cauliflower and potatoes together in a bowl with 3 tablespoons of oil, cumin seeds, and 1/4 teaspoon of salt.
3. Spread in hot baking pan and roast, stirring occasionally, until the cauliflower is tender and browned in spots and potatoes are just tender, about 20 minutes.
4.
While vegetables are roasting, cook the onion, garlic, jalapeño, and ginger in remaining 2 tablespoons of oil in a 12-inch heavy skillet over moderate heat, stirring frequently, until very soft and beginning to turn golden, 8-10 minutes.
5. Add ground cumin, coriander, turmeric, cayenne, and remaining 1/2 teaspoon of salt and cook, stirring constantly, 2 minutes.
6. Stir in water, scraping up any brown bits from the bottom of the skillet, then stir in roasted vegetable. Cook, covered, stirring occasionally, 5 minutes.
