Step 1: Preheat your oven to 400°F (200°C). Peel and cut the Russet potatoes into 1/2-inch thick chips. Rinse under cold water, then pat thoroughly dry. In a large bowl, toss the dried potatoes with 2 tablespoons olive oil, 1 teaspoon sea salt, and 1/2 teaspoon black pepper. Spread in a single layer on a baking sheet; bake for 25-30 minutes, flipping halfway, until golden brown and crispy.
Step 2: While chips bake, prepare the fish. Pat cod/haddock fillets completely dry. Season generously with salt and black pepper.
Step 3: Set up a dredging station: shallow dish with 1/2 cup flour; shallow dish with whisked egg and milk/water; shallow dish with 1 cup panko breadcrumbs combined with 1 tablespoon curry powder.
Step 4: Dip each fish fillet first in flour (shake off excess), then egg mixture, then panko-curry mixture, pressing firmly to coat evenly. Set aside.
Step 5: Heat 2-3 inches of vegetable oil in a heavy-bottomed pot to 350°F (175°C). Carefully lower fish fillets (one or two at a time) into hot oil. Fry for 4-6 minutes, flipping once, until golden brown and fish flakes easily. Transfer to a wire rack over paper towels to drain.
Step 6: Prepare the curry mayo: In a small bowl, combine 1/2 cup mayonnaise, 1-2 tablespoons curry powder, 1 teaspoon honey/maple syrup, and juice of 1/2 lime. Whisk until smooth. Taste and adjust seasonings.
Step 7: Assemble burgers: Lightly toast buns. Spread generous layer of curry mayo on both top and bottom buns. Place lettuce on the bottom bun, then the fried fish fillet, then a handful of chips, and optional pickled onions/gherkins. Top with the other bun.