Begin by heating your vegetable oil in a deep fryer or a large heavy pot. Aim for a temperature of about 350°F (175°C). Use a meat thermometer to check the temperature.
In a large mixing bowl, combine the flour, cornstarch, sugar, salt, baking powder, baking soda, onion salt, paprika, and ground black pepper. Whisk until evenly mixed. Slowly add the club soda, stirring until just combined.
While your oil is heating up, pat the fish pieces dry with paper towels.
Dip each piece of fish into the batter, ensuring it is fully coated. Allow excess batter to drip off before placing the fish into the hot oil.
Carefully lower the battered fish into the hot oil, working in batches. Fry for about 3-4 minutes on each side or until golden brown and cooked through.
Once cooked, transfer the fish to a cooling rack set over a baking sheet to drain excess oil. Serve hot with your favorite sides and dipping sauces.
