Trim the ends from the onion and peel away the outer skin.

Cut the onion crosswise into ¾- to 1-inch-thick slices, then separate the slices into rings, discarding the feathery skin between the rings.

In a small bowl, whisk together the garam masala, paprika, curry powder, and salt.

Place the flour, beaten eggs, and panko breadcrumbs in three separate shallow bowls.

Season each with one-third (2 tablespoons plus 2½ teaspoons) of the spice mixture.

Dip each onion ring in the spiced egg, dredge in the flour, then repeat with the egg and flour once more.

Dip the ring back into the egg again, then coat in the spiced breadcrumbs.

Repeat to coat all the onion rings and arrange them on a wire rack set over a baking sheet.

Place the onion rings in the freezer and chill until firm, at least 30 minutes.

Meanwhile, in a bowl, whisk together the mayonnaise, mint, lemon juice, and scallion.

Season with salt and refrigerate.

Preheat the air fryer to 375°F.

Spray 5-6 onion rings liberally with cooking spray and arrange them loosely in the air fryer basket, laying some flat and leaning some against the side of the basket.

Cook until the onion rings are tender and the breading is golden brown and crisp, about 10 minutes.

Season with more salt if needed.

Continue frying the remaining onion rings in batches.

Serve the hot onion rings with the mint-mayo dipping sauce and ketchup on the side.