Preheat your air fryer to 200°C (390°F) and prepare a mixing bowl. Wash and trim the green beans, then pat them dry thoroughly.
Toss the green beans in the bowl with a light spray of olive oil, ensuring they’re evenly coated without excess oil.
Sprinkle salt, black pepper, lemon zest, and chili flakes over the beans, then toss again to distribute the seasonings evenly.
Arrange the seasoned green beans in a single layer in the air fryer basket, making sure not to overcrowd for maximum crispness.
Roast the green beans at 200°C (390°F) for about 8-10 minutes, shaking the basket or tossing the beans halfway through to promote even browning and blistering.
Watch for blistered, slightly charred edges and a smoky aroma; they should be tender but crispy on the outside.
If the beans aren’t yet crispy enough, add a couple more minutes, checking frequently to prevent burning.
Remove the green beans from the air fryer and transfer them to a serving dish. While hot, sprinkle with freshly grated Parmesan or nutritional yeast if desired.
Finish by adding a final touch of lemon zest or a squeeze of lemon juice for extra brightness, then serve immediately while still crispy and smoky.