To make the pastry, mix the flour and icing sugar together in a bowl. Add the butter and rub it in lightly with your fingertips until the mixture resembles fine breadcrumbs. Alternatively, do this in a food processor or a mixer and then transfer to a bowl.
Mix the egg with the lemon juice and a tablespoon of water. Make a well in the centre of the flour mixture and pour in the egg mix. Using a table knife, work the liquid into the flour to bring the pastry together. If it seems too dry, add a splash more water. When the dough begins to stick together, use your hands to gently knead it into a ball.
Wrap in cling film and rest in the fridge for at least 15 minutes.
Preheat the oven to 200C/400F/Gas 6.
Roll out the pastry on a lightly floured surface to about a 3mm thickness and use it to line the tart tin, leaving excess pastry hanging over the edge. Keep a little uncooked pastry back in case you need to patch any cracks later.
Line the pastry case with baking parchment or foil, then fill with baking beans, or uncooked rice or lentils. Bake blind for 15 minutes, then remove the parchment and baking beans and return the pastry to the oven for approximately eight minutes, or until it looks dry and faintly coloured.
Using a small, sharp knife, trim away the excess pastry from the edge of the tin. Use a tiny bit of the reserved raw pastry to patch any cracks or holes if necessary. Turn the oven down to 190C/375F/Gas 5.
For the filling, put the both milks and the sugar in a large bowl or the bowl of a freestanding mixer. Beat together with an electric whisk, starting off slowly then increasing the speed, for around 10 minutes until the mixture is increased in volume, light and bubbly.
Pour the mixture into the pastry case. Bake for about 30 minutes (check after 20 minutes), or until just set and still a bit wobbly in the centre. Remove from the oven and leave to cool in the tin before slicing and serving.