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.

Using a fork to whisk continuously, and then add 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 30 minutes to 1 hour.

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.

Place the washed chips into a pan of cold water. Bring to a gentle boil and simmer for 3 to 4 minutes.

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 oil for about 2 minutes. Do not brown them. Once the chips are slightly cooked, remove them from the oil and drain. Keep to one side.

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. Dip into the batter, coating the entire fillet.

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.

Once cooked, remove the fillets from the hot oil and drain on paper towels. Sprinkle with salt. Cover with greaseproof paper 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.

Serve immediately with the hot fish accompanied by your favorite condiment.