Preheat your air fryer to 200°C (390°F) to ensure even cooking and crisping from the start.
Place the trimmed green beans in a mixing bowl and drizzle with olive oil, tossing gently to coat all the beans evenly. The beans should glisten lightly but not be greasy.
Sprinkle with salt and black pepper, then toss again to distribute the seasonings uniformly.
Spread the seasoned green beans in a single layer in the air fryer basket, making sure they are not overcrowded to allow air circulation for maximum crispness.
Roast the green beans for about 8 to 10 minutes, shaking the basket halfway through to ensure they cook evenly and develop a slight char on the edges.
Once the beans are tender and slightly charred, carefully remove the basket and transfer the green beans to a serving plate.
Squeeze fresh lemon juice over the hot green beans for brightness, and sprinkle with parmesan cheese if using. Toss gently to combine.
Serve immediately while crispy and hot, enjoying the smoky aroma and satisfying crunch with each bite.