Heat oven to 150C/130C fan/gas 2 and grease a 22cm loose-bottomed fluted tart tin. Put the butter in a bowl and beat with a wooden spoon until soft. Beat in the sugar, then stir in the lemon zest and ginger.
Stir in the flour and work with your hands to form a soft dough. Tip into the tin and press into an even, flat layer with your fingers. Prick all over with a fork and bake in the oven for 40 mins until pale gold. Cut into wedges then leave to cool completely in the tin. Remove from the tin and dust with caster sugar to serve.