Coat cauliflower: Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper. In a large bowl, toss cauliflower with 3 tablespoons cornstarch, salt, and pepper. Drizzle with 2 tablespoons oil, toss again, then add remaining 3 tablespoons cornstarch and 1 tablespoon oil. Toss until evenly coated.
Roast: Spread cauliflower on the prepared baking sheet in a single layer. Roast for 10 minutes, flip, then roast another 10–12 minutes until golden and tender.
Make the sauce: Meanwhile, in a small saucepan, combine ketchup, sugar, rice vinegar, soy sauce, and garlic. Bring to a simmer over medium heat.
Thicken: Stir the cornstarch slurry into the simmering sauce and cook for 1 minute, until glossy and thickened.
Combine: Transfer roasted cauliflower to a large bowl. Pour sauce over top and toss gently to coat.
Serve: Sprinkle with green onions and enjoy hot, with rice or as finger food.