Heat the peanut oil in a heavy-bottomed skillet or deep fryer over medium-high heat until it reaches 375°F.

Prepare the tartar sauce by mixing mayonnaise, Dijon mustard, lemon juice, diced dill pickles, capers, and fresh parsley. Season with salt and pepper.

Cut the cod into 4-6 oz portions, season with salt and pepper, and place on a wire rack.

Create the dry dredge by mixing flour with Old Bay seasoning. For the wet dredge, combine flour, additional Old Bay, baking powder, egg, and sparkling water.

Dredge the fish by first coating it in the dry dredge, then dipping it in the wet dredge.

Fry the dredged fish in batches for 2-3 minutes per side, or until golden brown, and drain on a wire rack.

Steam the hamburger buns until warm and soft.

Assemble the sandwich with tartar sauce, fried cod, cheddar cheese, and top bun.