Peel the onions and slice them into 1/4-inch thick rings. Separate the rings and set them aside.
In a bowl, mix the flour, baking powder, salt, and paprika (if using). In another bowl, whisk together the milk and egg.
Dip each onion ring into the flour mixture, ensuring it is well-coated. Shake off any excess flour. Then dip the floured ring into the milk and egg mixture, allowing any excess to drip off. Finally, coat the ring in breadcrumbs.
In a deep frying pan or a deep fryer, heat the oil to 350°F (175°C). You can test the oil by dropping a small piece of batter into it. If it sizzles and rises to the surface, the oil is ready.
Carefully place the breaded onion rings into the hot oil. Fry until they are golden brown and crispy. Remove the onion rings with a slotted spoon and place them on a paper towel-lined plate to drain any excess oil.
Serve the onion rings hot with your favourite dipping sauce. Common choices include ketchup, ranch dressing, or a spicy aioli.