In a large bowl, whisk together 1 cup of flour, cornstarch, baking powder, salt, and pepper. Gradually whisk in cold sparkling water until smooth. Let the batter rest while preparing the fish.

Prepare the fish

Pat haddock fillets dry with paper towels. Lightly dredge each fillet in extra flour and shake off the excess.

Fry the fish

Heat 1.5 liters of vegetable oil in a deep fryer or heavy-bottomed pot to 350°F (175°C). Dip each floured fillet into the batter, allowing excess to drip off, then carefully place into the hot oil. Fry for 5–7 minutes until golden brown and crispy, turning as needed. Remove with a slotted spoon and drain on paper towels.

Chips (Fries)

Prepare the potatoes

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

First fry

Heat oil to 325°F (160°C). Fry the potatoes in batches for 3–4 minutes until softened but not colored. Remove and drain on paper towels.

Second fry

Increase oil temperature to 375°F (190°C). Fry the potatoes again in batches for 2–3 minutes until golden and crispy. Drain and sprinkle with salt while hot.

Tartar Sauce

Make the tartar sauce

In a small bowl, combine mayonnaise, chopped pickles, capers, parsley, lemon juice, Dijon mustard, salt, and pepper. Mix well and refrigerate until ready to serve.

To Serve

Plate and garnish

Serve crispy haddock fillets with a side of golden fries. Add a dollop of tartar sauce and garnish with lemon wedges and chopped parsley if desired.