Slice the onion into thick rings and separate them. Set aside.
In the first bowl, mix the flour, baking powder, and salt.
In the second bowl, whisk together the egg and milk.
In the third bowl, place the bread crumbs.
Dip each onion ring into the flour mixture, then into the egg mixture, and finally coat it with bread crumbs.
Heat oil in a deep frying pan over medium heat.
Fry the onion rings in batches until golden brown, about 2-3 minutes per batch.
Remove and drain on paper towels.
Serve hot with ketchup, mayonnaise, or your favorite dipping sauce.