01 - Slice peeled onions into ½-inch thick rings. Separate into individual rings and discard the very small center pieces.
02 - Arrange three shallow bowls. In the first, combine flour, garlic powder, paprika, salt, and pepper. In the second, whisk eggs and buttermilk until blended. Place panko breadcrumbs in the third bowl.
03 - Dredge each onion ring in the flour mixture, shaking off excess. Dip into the egg mixture, allowing excess to drip off. Press firmly into panko breadcrumbs to coat thoroughly.
04 - Place breaded rings in a single layer in the air fryer basket, leaving space between each ring. Lightly coat tops with cooking spray.
05 - Cook at 400°F for 10–12 minutes, flipping halfway through cooking time, until golden brown and crispy. Work in batches if necessary to avoid overcrowding.
06 - Combine mayonnaise, ketchup, Dijon mustard, lemon juice, smoked paprika, cayenne, salt, and pepper in a small bowl. Mix until completely smooth. Refrigerate until serving.
07 - Transfer hot onion rings to a serving plate alongside the chilled dipping sauce. For best texture, serve immediately while crispiest.