Slice the onions as thinly as possible, then halve or quarter the rings so you have short pieces rather than long loops. Place them in a bowl, pour the buttermilk over, and soak for about 10 minutes.
In a shallow dish, whisk together the flour, salt, paprika, and garlic powder.
Lift the onions out of the buttermilk a handful at a time, letting the excess drip off, and dredge in the seasoned flour until well coated. Shake off any excess flour.
Arrange the onions in a single layer in the air fryer basket, working in batches as needed. Spray lightly and evenly with cooking spray until no dry flour is visible.
Air fry at 375 degrees F (190 degrees C) for 10 to 12 minutes, until golden brown. Start checking at 8 minutes and re-spray any dry patches.
Tip onto a plate, spread into a single layer, and salt while hot. The onions crisp further as they cool.