Make the Aioli: Blend all aioli ingredients except the oil in a mini food processor, gradually adding oil until thickened.
Prepare the Slaw: Combine all chopped slaw ingredients in a bowl and mix well.
Mix the Batter: Whisk batter ingredients in a bowl, adjusting with sparkling water or flour to get the right consistency.
Heat the Oil: Preheat oil in a deep pan to about 190°C (375°F).
Prepare the Fish: Season fish, lightly dust with flour, then dip into the batter, letting excess drip off.
Fry the Fish: Gently place fish in hot oil, frying for 4–5 minutes until golden and crispy. Drain on a wire rack and season immediately.
Toast the Buns: Lightly toast buns to prevent sogginess.
Assemble the Sandwich: Spread aioli on the bun base, add crispy fish, slaw, drizzle more aioli, and top with the bun. Serve and enjoy!