Half-fill a deep pan with frying oil and heat until you see a faint haze rising from the surface. To test if it's hot enough, drop in a small cube of bread—it should sink, rise back up, and turn golden in about 50 seconds.

While the oil heats, sift the self-raising flour and salt into a mixing bowl.

Beat in the egg, melted butter, and milk until you have a smooth, flowing batter with a consistency similar to heavy cream—thick enough to coat the back of a spoon but still pourable.

Dip two fish fillets into the batter, ensuring they're completely coated.

Using kitchen tongs or a fork, carefully lower the coated fish into the hot oil.

Fry until crisp and golden brown, about 6-8 minutes for larger pieces or 4-5 minutes for medium-sized fillets.

Lift the fish from the oil and drain on plain white paper to absorb excess oil.

Continue battering and frying the remaining fillets in batches of two until all are cooked.

Arrange the golden fish on plates with lemon wedges for serving.