Preheat the oven to 380 degrees F (190 degrees C). Slice onion into 3/4-inch thick rings.
Set up a breading station: Combine 1/2 cup flour and 1/2 teaspoon salt in a shallow bowl. Whisk together egg and milk in a second shallow bowl until light yellow and frothy. Combine panko, 2 tablespoons flour, remaining 1/2 teaspoon salt, and bbq rub in a third shallow bowl.
Dip the onion rings in the flour mixture and coat thoroughly. Next, dip onion rings into egg mixture, ensuring each ring is completely coated. Finally, dip each ring into panko mixture, pressing firming to adhere.
Place onion rings on a baking sheet and spray each one with nonstick spray. Place in the air fryer basket, working in batches if necessary.
Cook for 5 minutes, turn onion rings, and cook an additional 5 minutes on the other side. Serve immediately.