Mix together the flour, baking powder, turmeric and a pinch of salt in a bowl and make a well in the centre. Add half the measurement water to the well. Gradually whisk the flour into the water to make a smooth batter, then whisk in the remaining water.
Cut the potatoes into 1.5 cm (¾ inch) slices, then cut across to make chunky chips. Put them in a bowl of cold water.
Pat the fish dry on kitchen paper and cut into 4 portions. Season lightly and dust with extra flour. Thoroughly drain the chips and pat them dry on kitchen paper.
Pour the oil into a deep-fat fryer or large saucepan to a depth of at least 7 cm (3 inches) and heat to 180–190°C (350–375°F), or until a spoonful of batter turns golden in 30 seconds. Fry half the chips for 10 minutes, or until golden. Drain and keep warm while you cook the remainder. Keep all the chips warm while you fry the fish.
Dip 2 pieces of fish in the batter and lower them into the hot oil. Fry gently for 4–5 minutes, or until crisp and golden. Drain and keep warm while you fry the other pieces. Serve with the chips and some crushed minted peas (see