Preheat your oven to 450°F (230°C). Lightly grease a large baking sheet.
Make the cauliflower: Place the cauliflower in a large mixing bowl along with the oil, garlic, salt, and pepper. Toss well to evenly coat the cauliflower. Spread the cauliflower on the prepared baking sheet, making sure to space the florets out as much as possible. Bake for about 25 minutes, flipping halfway through, until the cauliflower is tender and browned.
Make the firecracker sauce: While the cauliflower is baking, place the brown sugar, hot sauce, water, cornstarch, rice vinegar, and red pepper flakes in a small pot. Bring to a simmer and cook for about 5 minutes, whisking often, until thickened.
Drizzle the sauce all over the roasted cauliflower on the pan, and toss well to evenly coat the cauliflower. Return the pan to the oven and continue baking for another 4–8 minutes, until the cauliflower is sticky-delicious. serve over rice and garnish with green onions.