Prepare the tartar sauce: Combine mayonnaise, Dijon mustard, ketchup, hot sauce, lemon juice, smoked paprika, garlic powder, cayenne pepper, salt, and pepper in a small bowl. Mix until smooth and set aside.

1. Prepare the fish fillets: Pat fish fillets dry with paper towels. In a shallow dish, combine flour, cornstarch, baking powder, garlic powder, paprika, salt, and pepper. Gradually add cold sparkling water and stir until the batter is smooth and thick. Heat vegetable oil in a skillet or deep fryer over medium-high heat (about 350°F or 175°C). Dip fish fillets into batter and fry for 3-4 minutes on each side until golden brown. Drain on paper towels.
2. Assemble the sandwich: Toast or steam the buns. Spread tartar sauce on the bottom half of each bun. Add a crispy fish fillet, top with a slice of American cheese (optional), and shredded lettuce (optional). Close the sandwich with the top half of the bun and serve immediately.