For the cod: Preheat vegetable oil in a deep-fryer to 375 degrees F. Then bread each medallion of fish with flour mixed with seafood seasoning. Tap off excess, and then dip in egg wash and finish in breadcrumbs, ensuring to coat evenly. After breading the fish, drop in the preheated oil and cook in batches until golden brown, flipping during the process. This should take 2 to 3 minutes. After frying, remove the fish from the oil, sprinkle with salt and pepper and place on paper towels. Allow the fish to drain excess oil.
For the sandwiches: Heat a nonstick pan over medium-high heat until warmed. During heating, with the softened butter, spread 1 tablespoon evenly on the top and bottom of 1 bun, and then toast in the pan until browned, about 2 minutes. After the first bun, reduce the heat to medium and repeat the process with the remaining buns.
To build the sandwiches, stack the bottom bun, spread tarter on the bottom bun, then add some lettuce, fish, pickles and finish with the top bun. Repeat with the remaining sandwiches.