Peel the 1 large sweet onion and slice into 1/3 to 1/2‑inch rings. Gently separate the rings and place them in a baking dish or a large resealable bag.

Pour 3 cups buttermilk over the onion rings so they are submerged. Cover and refrigerate for 4 to 12 hours to marinate.

When ready to cook, preheat the air fryer to 400°F.

Remove the onion rings from the buttermilk and drain off excess. Pat rings lightly with paper towels to remove excess liquid.

Prepare two shallow bowls: in the first bowl combine 1 cup panko or regular breadcrumbs, 3/4 cup yellow cornmeal, 1/2 cup all‑purpose flour, and 2 tablespoons Cajun seasoning; mix well. In the second bowl pour 1/2 cup egg whites and whisk until very foamy (rewhisk as needed if the foam deflates).

Working in small batches, dip each drained onion ring into the egg whites to coat, letting excess drip off, then dredge in the breadcrumb mixture, pressing lightly so the coating adheres. Place coated rings on a plate or rack.

Arrange 5 to 8 coated onion rings in a single layer in the air fryer basket without overlapping. Spray the tops thoroughly with oil spray.

Air fry at 400°F for 5 minutes. Open the basket, carefully flip the rings, spray the other side with oil spray, then continue air frying for another 5 to 7 minutes, until golden and crispy.

Transfer cooked rings to a wire rack or paper towel‑lined plate. Repeat steps 7–8 with remaining batches, rewhisking the egg whites if needed.

Serve the onion rings warm.