Heat the oil: If not using a deep-fryer, pour about 8 cups vegetable oil into a heavy, deep pot and heat to 360°F (182°C) using a candy or deep-fry thermometer.
Make the dry mix: In a large mixing bowl, whisk together the flour, cornstarch, sugar, salt, baking powder, baking soda, onion salt, paprika, and ground black pepper until evenly combined.
Add club soda: Pour the cold club soda into the dry ingredients and stir gently; the batter will foam—continue stirring until smooth and combined.
Coat the chicken: Dip each chicken tender into the batter to fully coat.
Fry the chicken: Carefully lower battered chicken into the hot oil and fry for about 2 to 3 minutes per side, or until the batter is golden and the pieces float to the surface.
Drain and rest: Remove chicken with a slotted spoon or tongs and place on a wire rack to drain; do not cover.
Finish: Sprinkle additional salt immediately after frying if desired and serve once slightly cooled.
