Peel the potatoes and cut them into thick fries. Rinse and soak them in cold water for at least 30 minutes to remove excess starch. Drain and dry thoroughly.

Make the Batter

In a large bowl, mix the all-purpose flour, cornstarch, baking powder, salt, and black pepper. Gradually add the cold sparkling water, whisking until the batter is smooth and has no lumps.

Heat the Oil

In a deep frying pan or pot, heat the vegetable oil to 350°F (175°C). Use a thermometer for accuracy.

Fry the Potatoes

Carefully add the fries to the hot oil in batches. Fry until golden brown and crispy, about 5-7 minutes. Remove with a slotted spoon and drain on paper towels. Season with salt.

Fry the Fish

Dip each cod fillet into the batter, allowing excess to drip off. Carefully place the battered fish into the hot oil and fry for about 4-5 minutes, or until golden brown and cooked through. Remove and drain on paper towels.

Serve

Serve the crispy fish with fries, lemon wedges, and tartar sauce on the side.