Start by trimming the green beans and rinsing them thoroughly under cold water, then pat dry until no moisture remains.
In a large bowl, toss the green beans with olive oil, salt, garlic powder, and black pepper, ensuring they are evenly coated.
Spread the seasoned green beans in a single layer in the air fryer basket, making sure they are not overlapping for even cooking.
Set the air fryer to 180°C (350°F) and cook the beans for 8 minutes, then open and shake or use tongs to turn them over.
Return the basket to the air fryer and cook for an additional 2 minutes, watching for the edges to develop a slight char and crispness.
Once done, carefully remove the green beans from the air fryer using tongs or a spatula and transfer to a serving plate.
Finish by squeezing fresh lemon juice over the beans and sprinkling with parmesan cheese if desired for added brightness and umami.
Serve immediately while the green beans are hot, crispy, and smoky, enjoying their vibrant flavor and satisfying crunch.