Rinse the Brussels sprouts and pat them completely dry with a clean towel.
Trim the stem ends and remove any yellow or loose outer leaves.
Cut large sprouts in half; quarter very large ones so they cook evenly.
Place the sprouts in a large bowl. Add olive oil, salt, black pepper, and garlic powder. Toss until evenly coated.
Arrange the sprouts in a single layer in the air fryer basket, cut-side down when possible. Do not overcrowd the basket.
Air fry at 375°F for 12–14 minutes, shaking the basket halfway through.
For extra browning, increase the temperature to 400°F for the final 2–3 minutes.
Transfer the Brussels sprouts to a serving bowl. Squeeze fresh lemon juice over the top.
Add Parmesan cheese, maple syrup, or parsley if desired. Serve immediately while hot and crispy.