Boil the potatoes until soft. Let them cool, then peel and mash well.
Wash and dry the big green chillies. Cut each one in half along its length and remove the seeds.
In a bowl, mix the mashed potatoes, onion, chopped green chillies, garlic, coriander leaves, black pepper powder, and salt.
Fill each chilli with the potato mix. Sprinkle chilli flakes on top.
Preheat the air fryer at 360°F (180°C) for 2 minutes.
Place the stuffed chillies in the basket one by one. Spray oil on top and air fry at 360°F (180°C) for 12 minutes.
Check the chillies. They are ready when the skin is soft and the filling is hot and lightly golden on top. Air fry for 2 to 3 more minutes if needed.
Serve hot as a snack or side dish.