In a small bowl, add the flour, flat beer, and 2 teaspoons of vegetable oil, mix together. Cover and place in the fridge for 1-2 hours.
While the flour mixture is chilling. Pour 2 inches of oil into a pot or large heavy skillet with high sides, heat to 185° C (365° F) on medium heat. Optionally, use a deep fat fryer.
Add potato wedges in batches to the oil, fry 4-6 minutes or until golden brown, turning once. Drain on paper towels and sprinkle with salt as desired. Allow the oil to return to 185° C (365° F) between batches.
Remove the flour mixture from the fridge, stir the egg yolk into the mixture.
Add the egg white to a medium mixing bowl, beat with an electric mixer at medium high speed until soft peaks form.
Fold egg white into the flour mixture.
I used pangasius for this, it is excellent whitefish, low cost and readily available. I cut the fillets in half for easier handling.
Return the oil to 185° C (365° F). Dip fish fillets into the batter in batches, fry for 4-6 minutes or until batter is golden brown and crispy, turning once. Drain on paper towels. Allow the oil to return to 185° C (365° F) between batches. Optionally, use a deep fryer, just leave the basket in the oil and carefully place 2 fillet halves at a time in the oil, when they turn golden brown they will float to the top, if you place in the basket then lower into the oil, the batter may brown around the basket and will result in some batter coming off when you remove them.
Serve with potato wedges, and lemon wedges if desired, or any chips you prefer. Enjoy.