Set aside 2 tablespoons of all-purpose flour.
In a large bowl, mix the remaining 5 tablespoons all-purpose flour along with 7 tablespoons cornstarch and 1 teaspoon baking powder.
Season lightly with a tiny pinch of fine salt and freshly ground black pepper.
Using a fork to whisk continuously, add ⅓ cup cold dark beer and ⅓ cup cold sparkling water to the flour mixture; continue mixing until you have a thick, smooth batter.
Place the batter in the refrigerator to rest for 30 to 60 minutes.
Meanwhile, cut 2-pounds peeled potatoes into a little less than ½-inch thick slices; slice these into ½-inch wide sticks.
Place the chips into a colander and rinse under cold water.
Place the washed chips into a saucepan with enough cold water to cover by about 1-inch.
Bring to a gentle boil over high heat; simmer on low heat for 3 to 4 minutes.
Remove from the heat; drain and transfer to a paper-towel-lined-plate; pat dry with paper towels.
Place in the refrigerator, covered with paper towels, until needed.
Meanwhile, lay the 4 white fish fillets on a paper towel and pat dry; season lightly with a little salt.
In a deep-fryer or large, deep saucepan, heat about 3-inches of vegetable oil to 350°F.
Par-cook the chips a few handfuls at a time in the oil; about 2 minutes ensuring they are not browned.
Remove the chips from the oil and drain on paper towels; set aside.
Place the 2 tablespoons of reserved flour into a shallow bowl; toss each fish fillet in the flour and shake off any excess.
Dip into the batter, coating the entire fillet.
Position a rack in the center of the oven and heat to 200°F.
Line a rimmed baking sheet with paper towels; set aside.
Bring the oil temperature back to 350°F.
Carefully lower each fillet into the hot oil.
Fry until the batter is crisp and golden, turning the fillets occasionally with a large slotted spoon; 6 to 8 minutes.
Remove the fillets from the oil to the paper-towel-lined baking sheet; sprinkle lightly with salt and transfer to the oven to keep warm.
Heat the oil to 400°F and cook the chips in batches until golden and crisp; about 5 minutes.
Remove from the oil and drain; season with salt.
Serve immediately, accompanied by your favorite condiment and a cold pint.