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To make the pastry, put the butter and caster sugar in a food processor. Add the egg and whizz for 30 secs, then tip in the flour and blitz for a few secs more until the dough just comes together – be careful not to overwork it.
Dust your work surface with flour, bring the dough together with your hands and lightly knead, shaping the pastry into a flat disc. Cover with cling film and chill in the fridge for 30 mins. Heat oven to 180C/160C fan/gas 4.
Roll out the pastry to £1 coin thickness and use to line a deep 23cm tart tin, then return to the fridge for 20 mins or until the pastry is hard. Line the pastry case with baking parchment, fill with baking beans and bake for 20 mins. Remove the baking beans and return to the oven for 5-10 mins until the base is golden and the sides are set.
Meanwhile, make the filling. Whisk the milk and sugar together in a stand mixer (or use an electric hand whisk) until light and fluffy, about 20 mins. Pour into the pre-baked pastry case and cook for 15 mins until risen and the surface is tacky. Remove from the oven and leave to cool and set before serving.
Pour the cream into a large bowl, add the lemon zest and lightly whip. Serve alongside the tart once cool.