Wash and trim the green beans, removing stem ends and any strings.
Pat beans dry with a kitchen towel to remove excess moisture.
In a large bowl, toss beans with olive oil, minced garlic, salt, and pepper until evenly coated.
Preheat the air fryer to 375°F (190°C) for 2–3 minutes.
Arrange beans in a single layer in the air fryer basket; do not overcrowd—work in batches if needed.
Air fry at 375°F (190°C) for 6 minutes, then shake or toss the basket.
Continue cooking for another 2–4 minutes until beans are blistered and tender-crisp.
While beans cook, toast sliced almonds in a dry skillet over medium heat until golden, if using.
Transfer cooked beans to a serving bowl, add lemon zest and lemon juice, and toss gently.
Sprinkle toasted almonds and red pepper flakes over the beans; adjust seasoning and serve immediately.