Combine mayonnaise, green onion, pickles, mint, dill, and capers; cover and refrigerate until ready to serve.

Pour oil to a depth of 3 inches in a large, heavy pot over medium heat; heat to 375ºF.

Combine ¼ cup flour and 1$ cup cornstarch in a large shallow dish. Whisk remaining flour, cornstarch, baking powder, and salt in a large bowl. Stir in club soda until mixture is the consistency of pancake batter.

Dredge fish fillets in flour/cornstarch mixture, tapping off excess. Working in batches of 2, dip fillets in batter, then carefully lower into oil. Fry 2 to 4 minutes per side, until golden. Transfer to a wire rack set over paper towels to drain; repeat with remaining fillets.

To serve, spread buns generously with tartar sauce, layer with lettuce, and top with fish fillets. Serve hot. 

Photo: © Brent Hofacker, stock.adobe.com