Preheat the 190°C.
Fry the potatoes in the hot oil until they are almost cooked. Drain them on paper towels.
Reduce the temperature of the oil to 170°C, dredge the fish in the batter, one piece at a time, and place them in the hot oil. Fry until the fish is golden brown. At the end increase the temperature to 190°C and fry the potatoes again for 1 to 2 minutes for added crispness.