1. Sieve the flour from a height on to a clean work surface. Using your hands, work the cubes of butter into the flour and sugar by rubbing your thumbs against your fingers until you end up with a fine, crumbly mixture. This is the point where you can spike the mixture with interesting flavours, so mix in your lemon zest.
2. Add the milk to the mixture and gently work it together till you have a ball of dough. Flour it lightly. Don’t work the pastry too much at this stage or it will become elastic and chewy, not crumbly and short. Flour your work surface and place the dough on top. Pat it into a flat round, flour it lightly, wrap it in clingfilm and put it into the fridge to rest for at least half an hour.
3. Preheat the oven to 180°C.
4. For the filling in a large bowl mix all together and set aside. 
5. Grease your tart/pie dish with butter or oil. Cut the dough in half and roll it out nice and thin until it can cover the dish. Line the dish with the pastry, ensure to neaten and trim around the edges. Pierce the base with a fork (this allows the base to cook nicely).
6. Add in all the filling on top of the lined dish. Brush the edges of the base with egg wash.
7. With the other half of pastry roll it out nice and thin. With a knife cut the rolled out pastry  into thin strips. Arrange them on top of the filling in a lattice pattern (or if not just lay the rolled out pastry on top and trim around the edges with a knife).
8. Brush the top with egg wash and sprinkle with a tablespoon of caster sugar. Bake the tart on the middle shelf of the oven for 35 to 40 minutes or until the filling is bubbly and the pastry is golden. Cool the pie to allow the filling to set, then serve.

Note:

Try to be confident and bring the pastry together as quickly as you can. Don’t knead it too much or the heat from your hands will melt the butter. A good tip is to hold your hands under cold running water beforehand to make them as cold as possible. That way you’ll end up with a delicate, flaky pastry every time.