Lightly salt the fish fillet and set it aside for 10 minutes to extract excess moisture. Then, pat the fish dry with a paper towel.
Prepare seasoned flour by mixing the all-purpose flour with pepper. Lightly coat the fish fillet in this flour to help the batter stick.
Prepare the batter by whisking together the all-purpose flour, rice flour, baking powder, salt, and cold sparkling water until smooth. Dip the fish into the batter, making sure it’s fully coated, and let it sit for a few moments to adhere well.
For the Chips, peel and cut the potato into thick fries (about 1 cm or ⅜ inch wide). Rinse the fries under cold water to remove excess starch, then pat them dry with a towel.
Heat oil in a deep pan or fryer to 325°F (160°C). Pre-fry the chips for 5-6 minutes until soft but not golden. Remove, drain on paper towels, and set aside.
Increase the oil temperature to 375°F (190°C) and fry the battered fish for 4-5 minutes until crisp and golden. Remove and drain on paper towels.
Return the chips again for 2-3 minutes until golden and crispy. Remove and drain on paper towels, then season with salt.
Plate the crispy fish alongside the chips and mushy peas. Add a drizzle of malt vinegar to the fish if desired, and serve with tartar sauce on the side if preferred.
