In a small bowl, combine flour, baking powder, and salt.
Dip the onion rings into the flour mixture until they are all coated; set aside.
Whisk the milk and egg into the flour mixture using a fork. Dip the floured rings into the batter to coat, then place on a wire rack to drain until the batter stops dripping.
Spread the bread crumbs out on a plate or shallow dish. Place rings one at a time into the crumbs, and scoop the crumbs up over the rings to coat. Give it a hard tap as you remove it from the crumbs. 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 salt, and serve.