Mix all the ingredients for the filling, except the butter together and set aside while making the pastry.
Place the flour and icing sugar together in a bowl and rub in the butter until the mixture resembles fine breadcrumbs.
Add the egg yolk and enough cold water to mix to bring the pastry together. Wrap and chill for 15 minutes.
Roll out two thirds of the pastry and use to line a 20cm/8in flan tin of pie plate. Pile the filing mixture into the pastry case and level the top. Dot with the butter.
Roll out the remaining pastry and cut into thin strips. Weave into lattice on top of the tart. Alternatively you can create the lattice on a sheet of baking parchment then transfer the lattice to the tart.
Stick the lattice to the tart with a little beaten egg white then brush the lattice work with beaten egg white to glaze. Chill for 15 minutes.
Preheat the oven to 200℃ (180℃ fan)/400°F/gas mark 6 and bake the tart in the centre of the oven for 15 minutes .
Reduce the oven temperature to 180℃ (170℃ fan)/400°F/gas mark 4 and bake for a further 15-20 minutes. Serve warm or cold.