Mix the flour and salt in a bowl. Gradually pour in the beer while mixing with a whisk, beat the batter until smooth. Cover the batter and leave to stand at room temperature for approx. 30 mins. Just before frying, beat the egg whites with the salt until stiff and fold carefully into the batter.
Rinse the potato sticks in cold water, pat dry. Fill a pot to ⅓ with oil and heat it to approx. 145°C. Add the dry potatoes in batches using a slotted spoon, fry for approx. 2 mins. Remove, leave to cool on a rack lined with paper towels. Turn the temperature up to approx. 180°C. Deep-fry the potatoes in batches for approx. 2½ mins. until crispy and golden. Remove, season with salt.
To deep-fry the fish
Salt the fish, dip in the batter in batches. Allow the excess batter to drip off, deep-fry for approx. 4 mins. until golden, turning occasionally. Remove, drain on paper towels, serve with the chips and mayonnaise.