Set aside 2 tablespoons of flour. In a large, roomy bowl, mix the remaining flour with the cornstarch and baking powder. Season as desired with salt and pepper. Mix flour with spices.

Using a fork to whisk continuously, add the beer and the sparkling water to the flour mixture and continue mixing until you have a thick, smooth batter. Place the batter in the fridge to rest for between 30 minutes and 1 hour.

Meanwhile, cut the potatoes into 1-centimeter slices (a little less than 1/2 inch), then slice these into 1-centimeter-wide chips. Place the chips into a colander and rinse under cold running water.

Place the washed chips into a pan of cold water. Bring to a gentle boil and simmer for 3 to 4 minutes.
Drain carefully through a colander then dry with paper towels. Keep in the fridge covered with paper towels until needed.

Meanwhile, lay the fish fillets on a paper towel and pat dry. Season lightly with a little sea salt.
Heat the oil to 350 F in a deep-fat fryer or large, deep saucepan. Cook the chips a few handfuls at a time in the fat for about 2 minutes. Do not brown them. Once the chips are slightly cooked, remove them from the fat and drain. Cook for another 3 minutes.

Dip the fish fillets in the batter and fry until golden brown. Remove from oil and dry with paper towel.