Preheat the oven to 200°C. Trim the Brussels sprouts and slice in half through the base.
Heat the vegetable oil in a cast iron pan over medium heat, and when the pan is nice and hot add the sprouts cut side down (if they don't all fit in at once, you can do this in batches, but they need direct contact with the pan to get nice and crispy).
Leave them for about 5 minutes until they get nice and dark brown, you can always take a peak under one to see how they are progressing.
Once the have nice brown bases move the pan to the oven and roast for around 15 minutes until they browned all over and tender in the middle.