Serve hot with lemon wedges and tartar sauce.
Remove fish when the coating is crisp and the fish flakes easily with a fork. Drain on a rack to keep it crunchy.
Fry the potatoes again at 375°F for 2–3 minutes until golden brown and crispy. Drain and immediately sprinkle with salt.
Dip each piece of fish into the batter, letting excess drip off. Carefully lower into hot oil and fry 5–6 minutes until deep golden and crisp.
In a bowl, whisk flour, cornstarch, baking powder, salt, and pepper. Stir in egg and cold sparkling water until the batter is smooth and slightly thick.
In a bowl, whisk flour, cornstarch, baking powder, salt, and pepper. Stir in egg and cold sparkling water until the batter is smooth and slightly thick.
Heat oil in a heavy pot to 325°F. Fry the potatoes in batches for 4–5 minutes until pale and tender but not browned, then drain on a wire rack.
Peel and cut the potatoes into thick fries. Rinse under cold water until clear, then pat completely dry with a towel.
