Preheat oven to 425 degrees and grease a baking sheet. Combine cauliflower and oil in a large ziplock bag. Seal and shake to coat cauliflower in the oil. Open the bag, add corn starch, seal bag and toss to coat again.
Transfer coated cauliflower to greased pan. Bake for 15-20 minutes until cauliflower starts to brown on the bottoms. Switch oven to broil and cook on high for 3-4 minutes just until tops start to brown. (watch carefully so the cauliflower doesn't burn!)
While cauliflower is cooking, prepare the sauce: Add sugar, vinegar, soy sauce, garlic powder, onion salt, and ketchup to a medium sauce pan. Stir and bring to a boil. In a small bowl whisk together the 1 tablespoon corn starch and cold water until dissolved. Add to sauce pan and stir until thickened, then reduce to low heat.
In a large heat-safe bowl combine roasted cauliflower and sauce. Serve with steamed rice and top with sliced green onions.
Serve over steamed rice or with your favorite fried rice.