Preheat oven to 180 degrees C / Gas 5. Grease a 20cm tart tin.
For the pastry: Combine butter with 30g icing sugar in a bowl until smooth; stir in 125g plain flour. Press dough into prepared tin; pierce crust with fork in several places to prevent crust from puffing up during baking.
Bake crust in the preheated oven until slightly golden brown, 10 to 15 minutes. Set aside to cool.
For the lemon filling:Whisk caster sugar and eggs in a bowl; stir 3 tablespoons plain flour into mixture and add lemon juice, lemon zest and salt. Whisk until thoroughly combined. Pour mixture into tart base.
Bake until filling is set, 20 to 25 minutes. Allow to cool and dust with icing sugar.