Preheat oven to approximately 200 C/400 F fan forced
Trim the brussel sprouts and cut in half lengthwise.
Toss the sprouts in the onion and garlic powder till well distributed and then stir through 1.5 tablespoons of the EVOO.
Spread out on a baking tray and cook in the oven for approximately 25-30 minutes, turning them halfway through cooking.
Just prior to them being cooked to your liking (less time for a less crispy result,) blend the balsamic vinegar with the remaining EVOO and drizzle over the sprouts.