Slice your fish into 2 portions and pat them down with paper towels until completely dry. Set them on a wired rack and salt the fish. Refrigerate for at least 30 minutes to draw out the moisture.
Tartare sauce: in a bowl, combine all the ingredients together. Set aside
Slaw: in a bowl, combined the shredded cabbage with enough mayo to lightly coat the cabbage. Set aside
Batter: in a bowl, combine all the ingredients together and whisk well until the batter is homogenous and combined.
Bring a pot of neutral oil to 180C/360F.
Take your fish out of the fridge and pat them dry again. Coat the fillets evenly in some plain flour and then dip them evenly into your batter. Carefully drop the coated fillet into the hot oil and cook 2 minutes on each side or until golden brown. Remove from oil and only a wired rack to drain.
To assemble, place some slaw onto a toasted bun, then top with the fish, tartare sauce and the top bun. Serve with chips!