Preheat oven to 450°F. Lightly grease large baking sheet.
In large bowl, combine cauliflower, oil, garlic, salt and pepper, tossing to coat. Arrange on prepared pan, spreading out evenly. Bake, flipping halfway through, until cauliflower is tender and browned, 25 minutes (see note #2)
While cauliflower is baking, combine brown sugar, hot sauce, water, cornstarch, rice vinegar and hot pepper flakes in small saucepan. Bring to a simmer over medium-low heat and cook, whisking often, until thickened, 5 minutes.
Drizzle sauce over roasted cauliflower, tossing to evenly coat. Return to oven and continue baking until cauliflower is sticky and delicious, 4 to 8 minutes.
Serve cauliflower over rice and garnish with green onions.