Peel the potatoes and cut them into thick fries. Rinse under cold water to remove excess starch, then soak them in a bowl of cold water for at least 30 minutes.

Make the Batter

In a large bowl, mix together the flour, cornstarch, baking powder, salt, and black pepper. Gradually whisk in the cold sparkling water until the batter is smooth and thick.

Heat the Oil

In a deep fryer or large pot, heat the vegetable oil to 350°F (175°C).

Fry the Potatoes

Drain the soaked potatoes and pat them dry with paper towels. Carefully add them to the hot oil in batches and fry until golden brown and crispy, about 5-7 minutes. Remove and drain on paper towels. Season with salt to taste.

Fry the Fish

Dip each piece of fish into the batter, allowing excess to drip off. Carefully place the battered fish in the hot oil and fry for about 4-6 minutes or until golden brown and cooked through. Remove and drain on paper towels.

Serve

Serve the crispy fish alongside the fries. Drizzle with malt vinegar and serve with tartar sauce on the side.