Half-fill a deep pan with melted fat or oil. Heat until a faint haze rises from it (or until a bread cube sinks to the bottom of the pan, rises to the top immediately and turns golden in 50 seconds).
Sift the flour and salt into a bowl.
Beat to a smooth batter, unbeaten egg, butter and milk. Then add the flour and salt mixture.
Coat 2 pieces of fish with batter.
Lift into the pan with a fork or kitchen tongs.
Fry until crisp and golden, allowing about 6-8 minutes for large pieces and 4-5 minutes for medium.
Remove from the pan and drain on absorbent kitchen paper.