1. Heat the oil in a deep-fryer to 365 degrees Fahrenheit (185 degrees Celsius).
2. Separate the onion slices into rings, and set aside. In a small bowl, mix together the baking powder, flour, and salt.
3. Dip the onion slices into the flour mixture until they’re well coated, then set aside. Whisk the egg and milk into the flour mixture using a fork. Dip the floured rings into the batter to coat, then place on a wire rack until the batter stops dripping.
4. Spread the breadcrumbs out on a plate or shallow dish. Place the rings, one at a time, into the crumbs, and scoop the crumbs up over the ring to coat. Give each ring a hard tap as you’re removing it from the crumbs. The coating should cling very well.
5. Deep fry your rings (a few at a time) for 2-3 minutes, or until golden brown. Place on paper towels to soak up any excess oil. Season with salt, and serve.