Sift the flours and salt into a bowl and put this in the fridge for 30 minutes. Make sure the water and sparkling water are very cold too.
Cut the fish into approx. 6cm pieces. Season them with salt and pepper.
Preheat the oil in a pan or in deep-fryer to 180°C.
Pour the water or crushed iced and sparkling water into a bowl. Add the egg or mayo, if used.
Add the flour and cornflour gradually, while mixing constantly with a wooden spoon. Don’t whisk or overmix it!
Test the oil with a drop of batter - if it sizzles and floats, it’s ready.
Coat the fish pieces first in some cornflour, then dip them in the batter.
Carefully put them in the oil and fry until crispy and light gold.
Place them on a plate, lined with paper kitchen towel.
Serve immediately with dips or sauces.