While your oil heats, make the tartar sauce by combining mayonnaise, fresh lemon juice, mustard powder, chopped capers, minced dill pickles, fresh dill, and black pepper in a small bowl. Stir until well combined and set aside. Pat the fish fillets dry with paper towels if not already done, and cut the potatoes into 1/2-inch batons. I like to soak the cut potatoes in cold water for 15 minutes before frying—this removes excess starch and gives you crispier chips with less greasiness.
Heat peanut or vegetable oil to 325°F in a deep pot or fryer. Working in batches to avoid crowding, fry the potato batons for 2-3 minutes until they're pale and cooked through but not yet golden. Remove with a slotted spoon and drain on paper towels. This first cook renders the potato while the second cook will create the crispy exterior. Increase the oil temperature to 375°F and fry the potatoes again for 3-4 minutes until they're golden brown and crispy. Transfer to a paper towel-lined plate, immediately season generously with Maldon sea salt, and keep warm in a 170-200°F oven while you prepare the fish.
In a large bowl, whisk together all-purpose flour, 1 cup rice flour, cornstarch, baking powder, onion powder, garlic powder, celery seed, sea salt, and cracked black pepper. Create a well in the center and pour in the ice-cold soda water, egg, and hot sauce. Gently whisk until just combined—the batter should be slightly lumpy and thick like pancake batter. Don't overmix, as this can develop gluten and make the coating tough instead of light and crispy. Keep the batter cold until you're ready to use it.
Heat peanut or vegetable oil to 375°F in a deep pot or fryer. Set up a dredging station: place the 1/2 cup rice flour in one shallow bowl and the batter from Step 3 in another. Working with one fillet at a time, coat it lightly in rice flour, then dip it completely into the batter, letting excess drip off. Carefully place the battered fish into the hot oil and fry for 4-5 minutes until the coating is golden brown and crispy. I always fry fish in smaller batches to keep the oil temperature consistent—overcrowding drops the temperature and results in greasy, soggy batter. Remove each fillet with a slotted spoon and drain on paper towels, then season immediately with sea salt.
Divide the golden fish fillets and crispy chips between serving plates. Serve with the tartar sauce from Step 1 on the side or spread lightly on the fish. Place a lemon wedge on each plate for squeezing over everything just before eating. Serve immediately while the fish is still hot and the chips are at their crispiest.