Wash and trim the green beans, snapping off the tough ends until they are bright and crisp-looking.
Place the green beans in a mixing bowl, then drizzle with olive oil. Toss gently to coat the beans evenly, watching them glisten with oil.
Squeeze fresh lemon juice over the beans and sprinkle with a pinch of salt. Toss again until the beans are well coated and fragrant with citrus.
Preheat your air fryer to 200°C (390°F) for about 3 minutes, ensuring it's hot enough for crisping.
Arrange the seasoned green beans in a single layer in the air fryer basket, avoiding overcrowding to promote even roasting.
Cook the green beans for 8-10 minutes, shaking the basket halfway through to turn them and ensure uniform crispness. Listen for the gentle sizzle and watch for golden edges to form.
Check the beans; they should be tender with crispy, slightly charred edges. If they need more crunch, roast for another 2 minutes, keeping an eye to prevent burning.
Remove the beans from the air fryer and transfer to a serving dish. Let them sit for a minute to firm up and develop their crispy texture.
Finish with a sprinkle of Parmesan or chopped fresh herbs if desired, then serve immediately for the best smoky crunch.