Wilt 75g spinach in a pan with a small amount of boiling water, then run under cold water to refresh, then squeeze out the moisture. Purée with a hand blender until smooth.
In a large bowl, beat 75g unsalted butter and 2 tbsp caster sugar together with an electric mixer until smooth, but not too fluffy and aerated. Add the spinach purée and beat well. Sift in 115g plain flour and mix gently.
Meanwhile, in a saucepan, whisk 4 large eggs and 200g granulated sugar until well combined, then add the 150ml lemon juice and 3 tbsp lemon zest and gently whisk until completely combined. Gradually sift in 40g plain flour, whisking gently to combine, and add 1 tbsp butter. Heat the mixture, stirring constantly, until thick and the butter has melted.
Spoon the mixture over the warm crust, spread evenly to the edges and bake for 25 minutes, or until the edges begin to turn golden. The filling will firm up as it cools. Leave to cool completely in the pan, then dust with 1 tbsp icing sugar and cut into squares to serve.