Cure the fish: by patting dry and seasoning with lemon zest, salt and pepper. Transfer to the fridge while you prep everything else.
Make the tartar sauce: by chopping all ingredients and adding to a bowl. Mix well until nice and creamy. Season to taste.
Make the slaw: by adding the mayo, dijon, sugar and vinegar to a bowl and mixing until combined. Chop cabbage and slice red onion. Add everything to a bowl and combine. Season to taste.
Prepare the fish: Add fry oil to a deep cast iron skillet or pan. Heat until temperature is 350 degrees.
Make beer batter: Add AP flour, rice flour, baking powder to a bowl. Add vodka and beer. Being careful to not overmix (you want some of those bubbles on the top of the batter) get a smooth light batter consistency.
Fry fish: When oil has reached 350, coat fish in beer batter and immediately add to the oil, carefully shaking the fish as you add it to the oil. Shaking allows the batter to break off slightly into those crispy bits. Cook for about 4 min per side. Transfer to a cooling rack and immediately sprinkle with flaky sea salt while it's still hot.
Assembly: Add butter to a skillet and toast your brioche buns. Add a layer of tartar sauce, a piece of crispy fish and a handful of slaw to your sandwich. Slice in half and enjoy!