Let's start with the batter. First, sift flour and add baking powder, garlic powder, salt, and pepper. Mix everything thoroughly.
Add one egg, mustard, and beer. Then mix the ingredients until you have a soft and consistent batter.
Heat oil in a stockpot.
In the meantime, cut the fish fillets in smaller rectangles, like fish fingers.
Soak the fillets in the batter. Dip them well, so they are completely covered.
Use a bit of batter to test the oil temperature. When the oil is hot enough, start to fry the fish.
Putting the fish inside hot oil requires extra care. I use tongs to avoid burning my hands. Avoid dropping or throwing the fish in the oil, since it will splash.
Don't overfill the stockpot as you won't have enough space to move the fish inside the oil. A stainless steel drainer is useful to remove floating bits of batter. Remove them carefully. If they burn, they will add an unpleasant burnt taste to the fish.
After 7 to 9 minutes the fish will be ready. Let the fish drying on paper towels for a few minutes.