Cut half the Brussels sprouts in half. Shave the rest of the Brussels by cutting them in half, laying the flat sides down, and then thinly slicing them. Set aside 1 cup of the shaved Brussels.
Add all the Brussels, except the ones you've set aside, to the other prepared tray, drizzle them with the remaining 1 ½ teaspoons of oil, and sprinkle sea salt on top.