Place a rack in the upper third of your oven and preheat the oven to 400 degrees F. Generously coat a rimmed baking sheet with nonstick spray (if doubling the recipe, place one rack in the upper third and one rack in the lower third and grease 2 baking sheets).
Trim off a little of the stem end of the Brussels sprouts, then with a mandolin or a sharp knife, carefully cut them into thin slices length-wise from tip to stem. If a few leaves come loose, discard any brown ones, but keep the larger green ones for roasting. They’ll be extra crispy and delicious. Place the sliced Brussels sprouts in a large mixing bowl as you go.
To the bowl, add the olive oil, garlic powder, salt, and black pepper. Toss to evenly coat the sprouts, adding a little oil if needed. You want them to be nicely coated so that they do not burn.
Spread the Brussels sprouts into a single layer on the prepared baking sheet. Bake for 10 minutes, then remove the pan from the oven and with a spatula, flip the sprouts and move them around to promote even browning. Spread them back into a single layer and sprinkle them with Parmesan (if using), then return to the oven and continue baking until they are browned and crisp, about 5 additional minutes (if using two pans, switch their positions on the upper and lower racks). Watch the chips very carefully towards the end to ensure they do not burn. Devour immediately, right from the pan, or transfer to a plate and then go for it!
TO STORE: Place leftover Brussels sprouts chips in an airtight storage container in the refrigerator for up to 4 days.
TO REHEAT: Gently rewarm leftovers on a baking sheet in the oven at 350 degrees F until hot. You can also reheat them in the microwave.
If you love crispy Brussels sprouts as much as we do, I recommend doubling the recipe, as these are very easy to devour! I can nearly finish a sheet pan by myself, and Ben and I can certainly polish off a single batch. Divide the Brussels sprouts between two baking sheets.