Wash and peel the potatoes if desired. Cut each potato into uniform steak-fries about 1/2–3/4 inch thick. Place cut fries into a large bowl of cold water and soak for 20–30 minutes to remove excess starch.

Dry and par-cook

Drain the potatoes and pat thoroughly dry with kitchen towels. Heat 2 cups of neutral oil in a heavy-bottomed pot to 325°F (160°C). Fry the fries in batches for 4–6 minutes until just tender and pale (do not brown). Remove and drain on paper towels.

Final fry

Increase oil temperature to 375°F (190°C). Fry the par-cooked fries in batches for 2–4 minutes until golden and crispy. Drain on paper towels, immediately season with kosher salt and freshly ground pepper, and toss with chopped parsley if using. Keep warm in a low oven (200°F/95°C) while frying fish.

House Cocktail Sauce

In a small bowl, whisk together ketchup, prepared horseradish, Worcestershire sauce, hot sauce, lemon juice, smoked paprika, and black pepper until smooth. Taste and adjust heat or lemon to preference. Refrigerate until serving.

Tartar Sauce

Combine mayonnaise, chopped dill pickles, chopped capers, minced shallot, lemon juice, Dijon mustard, chopped parsley, salt, and black pepper in a bowl. Stir until well blended. Chill at least 15 minutes to let flavors meld.

Fish

Season fillets

Pat the cod fillets dry with paper towels. Season both sides lightly with 1 tsp of the sea salt and a pinch of black pepper from the measured amounts.

Make batter

In a medium bowl, whisk together the remaining 1/2 cup flour, cornstarch, baking powder, remaining sea salt and black pepper. In a separate small bowl, beat the cold egg with the cold sparkling water and beer (if using). Pour the wet into the dry and whisk lightly just until combined — lumps are okay. The batter should be cold and thick enough to cling to the back of a spoon; if too thick, add 1–2 tbs more cold sparkling water.

Dust fillets

Place the 1 cup of all-purpose flour for dusting on a shallow plate. Lightly dust each seasoned fillet in the flour, shaking off excess. This helps the batter stick.

Heat oil

Pour vegetable oil into a deep skillet or pot to a depth of about 1.5–2 inches. Heat to 350–360°F (175–180°C) for frying battered fish. Use a thermometer for accuracy.

Batter and fry

Working one at a time, dip a floured fillet into the cold batter, letting excess drip off. Carefully lower into the hot oil. Fry in batches (do not overcrowd) for 4–6 minutes, turning once if necessary, until batter is puffed, golden-brown and internal fish temperature reaches 145°F (63°C). Remove to a wire rack set over a sheet tray to drain and stay crisp. Repeat with remaining fillets.

Serve

Plate each cod fillet with a generous portion of fresh-cut steak fries. Add a lemon wedge and small rameons of house cocktail sauce and tartar sauce. Serve immediately while hot and crispy.