Cut stem and root end from onions then remove skin of outer layer. Cut onion into ½" thick slices. Separate slices into individual rings then place in a large bowl.
In a separate medium-sized bowl, stir together flour, baking powder and seasoned salt until completely combined. Measure 1 Tbsp of flour mixture then sprinkle over onion rings. Toss to coat lightly then set aside.
Add beaten egg and milk to bowl of seasoned flour then whisk until smooth. Pour breadcrumbs onto a large plate. Dip each onion ring into flour batter, allowing excess to drip off. Dredge in breadcrumbs, coating completely, then place on platter.
Add oil to a deep fryer or large skillet deep enough to cover onion rings then heat to 350 degrees. Carefully place onion rings in the hot oil and fry until lightly golden brown on each side, about 2–3 minutes total.
Transfer crispy onion rings to a paper towel-lined plate to drain excess oil. Sprinkle with salt then serve while piping hot and crunchy!