Start by peeling the onions and slicing them into ¼-inch thick rings. Carefully separate the rings from each other and set aside.
In a mixing bowl, combine the flour, baking powder, and salt. In a separate bowl, whisk together the milk and egg until well combined. Gradually add the dry ingredients into the milk-egg mixture, whisking until smooth. If you want a spiced flavor, add a teaspoon of paprika to the batter.
Place breadcrumbs or panko crumbs in another shallow dish. Dip each onion ring into the batter, making sure it is evenly coated, then immediately dip it into the breadcrumbs to give it that crispy outer layer.
In a deep frying pan, heat about 2 inches of oil to 350°F (180°C). If you have a deep fryer, set it to the same temperature.
Once the oil is hot, carefully place the onion rings in the oil, making sure not to overcrowd the pan. Fry for 2-3 minutes, flipping halfway through, until they turn golden brown.
Using a slotted spoon, remove the onion rings and place them on a plate lined with paper towels to drain any excess oil. Serve hot with your favorite dipping sauce, like ketchup, ranch, or a zesty aioli.