Mix together the Parmesan, pepper and orange zest. Refrigerate until needed.
Halve the Brussels sprouts and remove any wilted outer leaves. Toss in olive oil and salt. Lay them cut side down on a baking tray without too many overlaps.
Roast them for anywhere between 20-35 mins until deeply browned and crispy. How long this takes largely depends on where in the oven you place them, how crowded your pan is etc. Keep an eye out after 20 mins.
Once the sprouts are browned to your liking and still very hot, toss them with 3/4 ths of the Pamesan-pepper-orange zest mixture. Sprinkle the remaining 1/4 over the top and serve hot.