Peel (optional) and cut the russet potatoes into 1/2–3/4 inch thick batons. Rinse the cut potatoes under cold water until the water runs clear to remove excess starch. Place potatoes in a large pot, cover with cold water by 1 inch, add 1 tsp salt, and bring to a boil. Simmer for 4–5 minutes until just tender but not falling apart (par-cooked). Drain and spread on a baking sheet to cool and dry; refrigerate for at least 15 minutes or up to a few hours to dry further.

First fry (blanch)

Heat vegetable oil for potatoes in a heavy pot or deep fryer to 325°F (160°C). Fry batches of potatoes for 4–5 minutes until pale and just softened but not browned. Use a slotted spoon to transfer fries to a paper towel-lined tray. Let them rest for at least 10 minutes (or chill).

Final fry

Raise oil temperature to 375°F (190°C). Fry potatoes in batches until golden-brown and crisp, about 2–3 minutes per batch. Drain on paper towels and immediately season with 1 tsp salt (and 1/2 tsp pepper if using). Keep warm in a low oven (200°F / 95°C) while you fry the fish.

Batter

Make beer batter

In a medium bowl whisk together 1.5 cups all-purpose flour, 1/4 cup cornstarch, 1 tsp baking powder, 1 tsp salt, and 1/2 tsp black pepper until evenly combined. Pour in 12 fl oz very cold beer in a steady stream, whisking to a smooth batter with the consistency of heavy cream. If desired, stir in 1/2 cup crushed ice to keep batter extremely cold (this helps crispness). Do not overmix; a few small lumps are fine. Rest batter in the fridge while heating oil (up to 15 minutes).

Fish

Prepare fish

Pat the cod fillets dry with paper towels. Season both sides with 1 tsp kosher salt and 1 tsp freshly ground black pepper. Lightly dust each fillet with about 1/4 cup all-purpose flour to help the batter adhere, shaking off excess.

Heat oil

Pour frying oil into a deep, heavy pot or deep fryer to a depth of 3–4 inches (use enough oil so the fish can float while frying). Heat oil to 350–375°F (175–190°C). Use a thermometer to maintain temperature.

Batter and fry fish

Working in small batches (2 fillets at a time, depending on pot size), dip each floured fillet into the cold beer batter, allowing excess to drip off. Carefully lower into hot oil. Fry for 5–7 minutes, turning once if necessary, until batter is deep golden brown and fish registers 145°F (63°C) internally or flakes easily. Remove with a slotted spoon and drain on a wire rack over a sheet pan or on paper towels. Keep warm in a low oven (200°F / 95°C) while frying remaining fillets.

Tartar Sauce & Finish

Make tartar sauce

In a small bowl combine 1/2 cup mayonnaise, finely diced dill pickle (or 2 tbsp sweet relish), 2 tbsp fresh lemon juice, 1 tsp chopped capers (optional), 1 tbsp chopped fresh parsley, 1/4 tsp salt, and 1/4 tsp black pepper. Stir until combined and refrigerate until ready to serve.

Serve

Plate 1–2 pieces of beer battered fish with a generous portion of fries. Add a dollop of tartar sauce and a lemon wedge. Optionally sprinkle a little extra chopped parsley over fries and fish. Serve immediately so batter remains crisp.