You can use any whitefish you'd like for this dish. Cod is a traditional choice although sole is a great choice as well. The ingredients can easily be doubled to make enough for 4 people, but will need to be fried in several batches. Larger batches of fish can be held in a warm oven on a cooking rack set over a baking sheet, or better yet, eat the batches as you fry them. The cooking oil can be cooled, strained, poured back into the bottle, and used 2 or 3 more times for frying fish. If possible, make the tartar sauce a day ahead to let the flavours develop.
FOR THE TARTAR SAUCE
Mix the mayonnaise with the cornichon, shallot, dill, capers, vinegar and lemon juice to taste. Refrigerate for at least 2 hours, or preferably overnight, to allow for the flavours to develop.
FOR THE BATTER
Mix the self-raising flour, cornflour, baking powder and salt in a medium bowl. Stir in the beer. The batter should be thick and somewhat lumpy. Let the batter sit for at least 10 minutes or up to 2 hours.
On a mandoline, benriner, or by hand, thinly slice the potatoes 1/16-inch thick. Place in a large bowl of cold water.
Place the oil in a Copper Core Chef’s Pan and heat the oil over medium-high heat to 325°F.
Drain the potato slices and dry well on paper towels. Add about half of the potato slices to the hot oil. The temperature will fall, but will rise as the potatoes cook. Do not overcrowd the pan. Stir with a slotted spoon to brown the chips evenly. As the chips turn a rich golden brown, about 10 to 12 minutes, remove them to the rack to drain and sprinkle with salt. Repeat with the remaining potatoes.
Increase the heat to medium-high until the oil temperature reaches 350°F. Adjusting the heat as necessary to keep the oil at 350°F while frying.
Cut each fish filet in half on the diagonal.
Dip a piece of fish into the batter. Hold the filet vertically over the bowl of batter and run your fingers down the sides of the filet to leave just a thin coating of batter, allowing excess batter to run back into the bowl. Gently lower the fish into the oil and repeat, adding half the fish filets to the oil. Move the filets with a slotted spoon, to allow them to brown evenly. Cook the filets for about 2 to 3 minutes, turning the filets over halfway through the cooking until the batter is crisp and a rich golden brown. As the pieces are cooked, remove them to the rack to drain.
Slice the parsley into a fine julienne. Arrange the fish filets on serving plates with a stack of the chips. Sprinkle the fish and chips with the parsley, salt and pepper. Serve immediately.