Set oven to 400/F or Mark 6. Line a 18 cm flan dish with the pastry. Put the lemon rind and juice, butter and sugar into a saucepan and heat gently until the sugar has completely dissolved, stirring as little as possible. Allow the mixture to cool completely, then strain the beaten eggs into it and stir gently until combined. Pour the mixture into the pastry case, and brush the edges of the pie with milk to glaze. Cook for 10 minutes, then reduce the oven temperature to 350/F or Mark -20ºC or a further 15 to 20 minutes until the filling is set and the pastry lightly golden. Serve hot or cold with cream.