In a small bowl, combine mayo, capers, chopped pickles, lemon juice, lemon zest, dill, salt, and pepper. Mix well and set aside.
In a separate bowl, combine flour, corn starch, and spices. Gradually mix in egg, then whisk in beer or sparkling water until a thin batter forms.
Salt both sides of the fish with 1 tbsp of salt and slice into portions that fit inside your buns.
Heat up enough oil in a deep pan or fryer, until375°F for frying. it reaches 350-
Dredge each piece of fish in flour, dip in batter, and fry in hot oil for 3-4 minutes on each side until golden brown.
Slice your buns and add a slice of American or Cheddar cheese to the bottom half of each bun. Toast the buns in the oven or toaster oven until the cheese is melted and the buns are slightly crispy.
To assemble the sandwich, spread the sauce on the top half of each bun. Place a piece of crispy fish on top of the cheese on each bottom bun. Add some lettuce and tomato if desired, then place the top bun on top and press down gently.