Step 1

Heat the oven to 475°F with a rack in the middle position. In a small bowl, whisk together the ½ cup cornstarch, garam masala and 1 teaspoon each salt and pepper.

Step 2

In a large bowl, toss the cauliflower with the ⅓ cup oil, then use your hands to rub the oil into the florets. Sprinkle with the cornstarch mixture and toss, pressing the cornstarch into the cauliflower, until evenly coated. Transfer the florets to a rimmed baking sheet, shaking off excess cornstarch and turning the pieces cut side down as much as possible. Roast for 20 minutes.

Step 3

Meanwhile, in a small bowl, whisk together the remaining 1 teaspoon cornstarch with ¼ cup water until smooth. Stir in the ketchup, soy sauce, vinegar and ½ teaspoon pepper; set aside.

Step 4

Using a thin metal spatula, flip the cauliflower, then continue to roast until deep golden brown, another 20 to 25 minutes. Scrape the cauliflower into a clean large bowl.

Step 5

In a small saucepan over medium-high, heat the remaining 1 tablespoon oil until shimmering. Add the chilies, scallion whites, ginger and garlic. Cook, stirring often, until the mixture browns, about 3 minutes. Whisk the ketchup mixture to recombine, then pour it into the pan. Bring to a boil, whisking constantly; the sauce will thicken slightly.

Step 6

Immediately pour the sauce over the cauliflower, add the scallion greens and toss. Taste and season with salt and pepper. Transfer to a serving dish and top with the cilantro.