Preparing the Fish

1. Select the Fish: Choose fresh, firm white fish fillets. Pat them dry with paper towels to remove excess moisture.
2. Mix the Batter: In a bowl, combine the flour, cornstarch, baking powder, and salt. Gradually add the cold sparkling water, whisking continuously until you achieve a smooth, lump-free batter. Refrigerate the batter for about 30 minutes.
3. Heat the Oil: In a deep frying pan or fryer, heat vegetable oil to 350°F (175°C).
4. Coat the Fish: Dip each piece of fish into the batter, ensuring it is fully coated.
5. Fry the Fish: Carefully place the battered fish into the hot oil. Fry for 4-6 minutes or until the batter is golden brown and crispy. Remove and drain on paper towels.

Preparing the Chips

1. Peel and Cut the Potatoes: Peel the russet potatoes and cut them into thick strips.
2. Soak and Rinse: Soak the potato strips in cold water for about 30 minutes. Drain and rinse them thoroughly.
3. First Fry: Heat vegetable oil to 325°F (160°C). Fry the potatoes in batches for 3-4 minutes, until they are soft but not colored. Remove and drain on paper towels.
4. Second Fry: Increase the oil temperature to 375°F (190°C). Fry the chips again until they are golden and crispy. Drain on paper towels and season with salt.

Making the Tartar Sauce

1. Combine Ingredients: In a bowl, mix the mayonnaise, dill pickles, capers, lemon juice, and mustard.
2. Season: Add salt and pepper to taste. Stir until all ingredients are well combined. Refrigerate until ready to serve.