Heat oil in a pan for deep-frying.Separate the onion slices into rings, and set aside. In a small bowl, stir together the flour, baking powder and salt. Dip the onion slices into the flour mixture until they are all coated; set aside.
Whisk the egg and milk into the flour mixture using a fork. Dip the floured rings into the batter to coat.Spread the bread crumbs out on a plate. Place rings one at a time into the crumbs, and scoop the crumbs up over the ring to coat The coating should cling very well. Repeat with remaining rings.
Deep fry the rings a few at a time for 2 to 3 minutes, or until golden brown. Remove to paper towels to drain. Season with seasoning salt, and serve.