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