Stir the mayonnaise, pickles and lemon juice together and refrigerate the tartar sauce.
Pat the cod dry, season it with the salt and coat each piece first in flour, then beaten egg and finally panko.
Heat the oil in a deep skillet to 350°F and keep the pan no more than halfway full.
Fry 2 fillets at a time for 3 to 4 minutes per side, maintaining 340°F to 350°F, until deeply crisp and the centers reach 145°F.
Drain the fish on a wire rack and toast the buns cut-side down in a dry skillet for 1 minute.
Layer each bottom bun with lettuce and one cod fillet, spread tartar sauce on the top bun and close the sandwich.