Preheat the oil in a large pot to 300 degrees F. Preheat the oven to 350 degrees F. Line a tray with paper towels.
Cut the potatoes into large wedges, about 10 per potato. Fry the potatoes for around 4 minutes. (They will be soggy.) Remove them to the lined tray and pat them dry. Turn the oil to 375 degrees F.
Meanwhile, sprinkle the fish all over with salt. Combine 1/2 cup flour and 1 teaspoon turmeric on a plate, then season with salt. Dredge the fish in the flour mixture and let sit on the plate. Mix the baking soda with the remaining 2 cups flour and 2 teaspoons turmeric in a second bowl. Season well with salt. Slowly whisk in the beer until the batter has a pancake-batter consistency (you might not use it all). Fold in the egg whites. Adjust with more or less flour or beer as needed.
Line a sheet tray with a wire rack. Fry the potatoes at 375 degrees F until crispy, about 4 minutes. Transfer to the rack and season with salt immediately. Hold the chips in the oven while you fry the fish.
Lower the fry oil to 250 degrees F and line a plate with paper towels. Pat the excess flour from the fish, then coat in the batter. Gently lower the fish into the oil and fry until golden brown and the fish is cooked through, 3 to 4 minutes. Transfer the fish to the lined plate and season with salt. Serve with Tartar Sauce, chips and lemon wedges.
Combine the mayonnaise, pickles, capers, peppers, lemon zest, lemon juice and dill in a medium bowl. Season with salt and pepper and mix until combined.