Heat oil in a deep fryer to 365 degrees. Separate onion slices into rings. In a large bowl stir together flour, baking powder, and salt.
Dip the onion ring slices into the flour mixture until they are well coated. Set aside.
Whisk the eggs and milk into the flour mixture until a batter has formed. Dip the floured onion rings in the batter to coat.
Place the coated onion rings on a wire rack until the batter stops dripping. You can place a sheet of aluminum foil under the wire rack to catch the drips. Pour the breadcrumbs into a shallow bowl or plate.
Coat the onion rings with the bread crumbs until coated thoroughly.
Deep fry onion rings in batches for 2-3 minutes or until golden brown. Drain on paper towels. Season with seasoning salt.