Preheat an oven to 100C and place a try inside for keeping the fish hot while you cook the chips (or vice versa).
Place the oil into a large pot, ensuring that the oil doesn’t come up more than halfway up the pot.
Place the pot on medium-high heat and monitor the temperature until it reaches 180℃.
Combine the flour, cornflour, baking powder and salt in a medium-sized bowl and whisk to combine.
Pour in the cold lager and whisk lightly until smooth.
Layout a plate and add your additional flour and season lightly with salt. Have a tray lined with absorbant kitchen towel set next to your pot also.
Season the fillets of fish with salt on both sides and dust well in the flour, shaking off any excess.
Working with one fillet at a time, from the tail end of the fish, dredge it through the beer batter, allowing any excess to drip off.
Gently place your fish in the hot oil, swaying it into the pot away from you to avoid any unnecessary splashes. Allow to cook for 2 minutes or until browned before flipping it over to continue browning the second side for another 2 minutes.
Once browned, les the excess oil drain off from the fish by placing it on a tray lined with absorbant paper towel. Then place into the oven until you are ready to plate.
Remember to turn off the heat to your pot of oil once done frying. If you will be cooking more portions of fish, make sure your oil is at a steady 180℃ before you add more fish.