Gather all ingredients.

Slice onion into ¼-inch-thick rings. Heat oil in a deep-fryer to 365 degrees F (185 degrees C). Place a wire rack over a sheet of aluminum foil.

Prepare breading station by setting out 3 wide, shallow dishes. Whisk flour, baking powder, and salt together in the first dish. Whisk milk and egg together in the second dish. Place bread crumbs in the third dish.

Dip each onion ring into the flour mixture, turning several times until fully coated with flour.

Transfer to the egg mixture and use a fork to turn until coated. Lift onion with the fork and shake gently so excess liquid drips back into the dish.

Place onion in the bread crumbs and turn several times to coat, scooping crumbs over the ring if necessary.

Lift again with the fork, tap any excess bread crumbs back into the dish, and place on the wire rack while you prepare the remaining onion rings.

Deep-fry 3 to 4 onion rings at a time in the preheated oil until golden brown, 2 to 3 minutes. Drain on paper towels while you deep-fry the remaining rings.

Sprinkle with seasoning salt before serving.