Spicy Roasted Cauliflower Stir-Fry

Arrange a rack in the middle of the oven and heat to 450°F. Line a rimmed baking sheet with parchment paper. Place the cauliflower on the baking sheet, drizzle with 2 tablespoons of the oil and season with salt and pepper. Spread into an even layer and roast until crisp and golden, stirring halfway through, about 30 minutes total. Heat the remaining 2 tablespoons of oil in a wok over medium-high heat until shimmering. Add the ginger and stir-fry until caramelized, 1 to 2 minutes. Add the onion and garlic, and cook for 2 minutes more, making sure they don't burn. Turn up the heat to high, then add the roasted cauliflower and scallions. Stir-fry for 30 seconds. Add the Shaoxing rice wine or sherry, tamari or soy sauce, sesame oil, red pepper flakes if using, and sugar. Stir-fry for about 1 minute, then serve with steamed rice if desired.