Prepare the potatoes. Cut the potatoes into thick fries about ½-inch wide. Place them in a bowl of cold water and soak for at least 20 minutes to remove excess starch, which helps them crisp up nicely when fried.

Preheat the oil. Fill a large heavy pot or Dutch oven with about 2–3 inches of oil and heat over medium-high heat until it reaches 325°F.

Par-cook the fries. Drain and thoroughly dry the potatoes with a clean towel. Fry them in batches for about 4–5 minutes until tender but still pale. Transfer the fries to a paper-towel-lined plate and set aside. Increase the oil temperature to 375°F.

Mix the beer batter. In a medium bowl, whisk together the flour, cornstarch, Old Bay seasoning, garlic powder, baking powder and salt until evenly combined. Slowly whisk in the cold beer until a smooth batter forms that is thick enough to coat the back of a spoon.

Season and dredge the fish. Pat the fish dry with paper towels and season lightly with the Old Bay, salt and pepper. Dredge each piece in the flour, shaking off the excess so the batter will adhere evenly.

Batter and fry the fish. Dip each piece of fish into the beer batter, letting the excess drip off, then carefully lower it into the hot oil. Fry for about 4–5 minutes, turning once if needed, until the batter is deeply golden and crisp and the fish flakes easily. Transfer the cooked fish to a wire rack or paper-towel-lined plate.

Finish frying the chips. Return the par-cooked fries to the hot oil and fry for another 2–3 minutes until golden and crispy. Transfer to a paper-towel-lined plate and immediately sprinkle with flaky sea salt and a light pinch of Old Bay.

Serve. Serve the hot beer-battered fish sprinkled alongside the fries with lemon wedges, tartar sauce and malt vinegar. Sprinkle with chopped parsley if desired and enjoy right away while everything is crisp and hot.