Wash fish fillets and pat dry. Sprinkle both sides generously with Cajun or Creole seasoning. Spread flour on a plate; whisk eggs in a shallow bowl until no thick strands of egg white remain; spread panko on another plate.

Dredge fish fillets in flour, shaking off excess; dip in eggs, then in panko bread crumbs, pressing to adhere. Place on a wire rack and refrigerate for 20 minutes.

Place oil and butter in a large skillet over medium-high heat; oil is hot when a pinch of bread crumbs sizzle vigorously when dropped into oil. Add fish fillets and cook until golden brown, 2 to 3 minutes, rotating after 1 minute. Turn over and cook until golden brown, 2 to 3 minutes. Drain briefly on a rack set over paper towels; do not place directly on paper towels to avoid a soggy crust. 

If desired, lightly toast buns; spread the bottom bun with Dill Pickle Tartar Sauce and garnish with lettuce. Top with a fish fillet, additional tartar sauce, and garnish as desired.