Set aside 2 tablespoons of flour. In a large, roomy bowl, mix the remaining flour with the cornstarch and baking powder. Season lightly with a tiny pinch of salt and pepper.
Mixing 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.
Adding beer to the flour mix
Meanwhile, cut the potatoes into a little less than 1/2-inch-thick slices, then slice these into 1/2-inch-wide chips. Place the chips into a colander and rinse under cold running water.
Cut potatoes for chips in a colander
Place the washed chips into a pan of cold water. Bring to a gentle boil and simmer for 3 to 4 minutes.
Par-boiling the cut potatoes
Drain carefully through a colander, then dry with paper towels. Keep in the fridge covered with paper towels until needed.
Drying the potatoes on paper towels
Meanwhile, lay the fish fillets on a paper towel and pat dry. Season lightly with a little sea salt.
Drying fish on paper towels
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. Keep to one side.
Frying the potatoes
Place the 2 tablespoons of flour reserved from the batter mix into a shallow bowl. Toss each fish fillet in the flour and shake off any excess.
Coating the fish with flour
Dip into the batter, coating the entire fillet.
Dipping the fish into the batter
Check that the oil temperature is still 350 F. Carefully lower each fillet into the hot oil. Fry for approximately 8 minutes, or until the batter is crisp and golden, turning the fillets from time to time with a large slotted spoon.
Frying the fish
Once cooked, remove the fillets from the hot oil and drain on paper towels. Sprinkle with salt. Cover with greaseproof paper (parchment paper) and keep hot.
Setting the fried fish on paper towels to drain
Heat the oil to 400 F, then cook the chips until golden and crisp, or about 5 minutes. Remove from the oil and drain. Season with salt.
Potatoes are browning as they fry
Serve immediately with the hot fish accompanied by your favorite condiment.