Set your air fryer to 375°F (190°C) and let it preheat for around 5 minutes.
Trim off the ends and remove any discolored outer leaves. Cut the Brussels sprouts in half for even cooking.
Place the sprouts in a large bowl. Add olive oil, salt, pepper, lemon zest, garlic, and balsamic vinegar. Mix well until everything is coated evenly.
Arrange the sprouts in the air fryer basket in a single layer. If needed, cook in batches to avoid overcrowding.
Air fry for about 15 minutes. Shake the basket or stir halfway through for even browning.
Use a fork to check if the sprouts are tender on the inside and crispy on the outside. Add 2–5 more minutes if needed.
Once ready, take them out and sprinkle with Parmesan cheese if you like. These Brussels sprouts can be served as a side or a light snack.