For the Batter: Whisk together flour, cornstarch, beer, egg, and salt in a medium-sized bowl. Make sure there are no lumps. Cover bowl and place in fridge for at least 20 minutes, and up to 3 hours.
For the Fish: Pour oil into a large dutch oven. Turn heat to medium and bring to a temperature of 375°F.
Dry fish fillets with paper towels. Season both sides with salt and pepper. Drop each piece into the bowl of batter.
When oil is at 375°F., remove one piece of fish out with a pair of tongs, letting the excess batter drip back into the bowl. Then lower it into the oil, holding on to it for a few seconds to keep it from sticking to the bottom. Repeat process with other fillets. Adjust heat to medium-high to keep temperature at about 350°F. Cook until fillets are golden brown, about five minutes.
Drain fish fillets on a baking sheet covered with paper towels. Serve with coleslaw, malt vinegar, lemon, or whatever you like best.
Mothers Secret Recipes of Comfort Food & Culinary Inspiration from an Organic and Allergen-free Approach