For the caper lemon tartar sauce: Mix the mayo, capers, relish, lemon juice, vinegar and Worcestershire sauce in a bowl. Season with salt and pepper, cover and refrigerate for 15 minutes marry the flavours.
For the fennel slaw: Mix the red cabbage, parsley and fennel in a large bowl. Whisk the lemon juice and mustard in a small bowl; whisk in the oil until the dressing is smooth. Toss with the fennel mixture and season with salt and pepper. Slaw can be stored in an airtight container and refrigerated up to 2 days.
For the fish: Heat the oil to 176°C in a heavy-bottomed Dutch oven or deep fryer.
Whisk 400g flour, baking powder and salt into a bowl. Add the beer and egg and mix until combined.
Sprinkle the fish with salt and pepper. Dredge the fish in the remaining cup flour, shake off any excess, then dunk into the batter mixture, letting the excess drip off. Fry the fish in the hot oil until golden brown and crispy, 4 to 5 minutes.
For the sandwich build: Place the hot, fried fish on the rolls. Then top with the slaw and slather the top bun in the tartar sauce. Garnish with a fennel frond. Grab with both hands and bite hard.