Preheat your air fryer to 200°C (392°F). Set it to warm up while you prepare the green beans.
Trim the ends off the green beans and pat them dry with a clean towel to remove excess moisture.
Place the green beans in a mixing bowl, then drizzle with olive oil, tossing gently to coat each bean evenly.
Sprinkle with salt and black pepper, then toss again to distribute the seasonings well.
Spread the coated green beans in a single layer in the air fryer basket, making sure they aren’t overcrowded for even crisping.
Cook the green beans for 8-10 minutes, shaking the basket halfway through to promote even browning and crispness.
Check the beans: they should be bright green with crispy, browned edges and a satisfying crackle when pressed.
If they aren’t quite crispy enough, cook for another 2-3 minutes, keeping an eye to prevent over-browning.
Once done, transfer the green beans to a serving dish, then sprinkle generously with lemon zest and Parmesan cheese if using.
Serve immediately while hot and crispy for the best flavor and texture.