Preheat the oven to 425 degrees . In a bowl, pour the melted butter over the panko and work it into the crumbs to evenly distribute. Season with the Old Bay, onion powder, garlic powder, lemon peel, thyme and chives.
Season the cod with salt and pepper, then coat in the flour, egg whites and panko mixture. Arrange on a broiler pan or cooling rack placed on a baking sheet. Bake until deeply golden and crisp, about 20 minutes.
Place the english muffin bottoms or 4 muffin bread slices on a work surface. Layer with the slaw or lettuce and then the fish. Dress the fish with the lemon juice or malt vinegar. Top with the pickles, onion and muffin tops or remaining 4 bread slices.