In a deep fryer or a large pan, heat a large amount of oil (or beef fat) at 260 F / 125 C.
Fry the French fries (a few handfuls at a time) in the fat for a few minutes. Do not brown them. Once the French fries are lightly cooked, remove them from the fat and drain them. Set aside.
Dip each fish fillet in the batter. Then carefully place each fillet in the hot oil. Fry for about 8 to 10 minutes or until crisp and golden brown, turning fillets occasionally.
Heat the reserved oil to 350 F / 175 C then fry the French fries again until they are golden and crisp, about 5 minutes.