Grease and line a 20cm smooth edged tart tin.
Using your fingers, make the pastry by rubbing the butter into the flour with the lemon zest and a pinch of salt until it resembles breadcrumbs.
Add the sugar, egg and milk and bring together to form a dough.
On a lightly floured surface, roll the pastry out and layer into the tin.
Leave a 2cm pastry overhang around the edges.
Chill for 30 mins.
Preheat the oven to 180°C / 350°F / Gas mark 4.
Line the tart tin with greaseproof paper and baking beans.
Bake blind for 15 minutes, then remove and discard the beans and continue to cook for a further 10 minutes until the base has a biscuit look.
Remove from oven and reduce the temperature to 140°C / 275°F / Gas mark 1.
In a saucepan, bring the cream, milk, vanilla pod, lemon zest and a small grating of the nutmeg to the boil.
Beat the egg yolks with the sugar until pale and golden then pour the hot milk and cream over, beating as you go.
Strain the custard into a jug and allow it to set for a few minutes, then skim off any skin or froth.
Remove from the oven, trim the pastry edges off using the knife to trim away from the pastry.
Gently pour the custard into the tart case, grate some more nutmeg over the top and bake for 20 minutes or until just set with a slight wobble in the middle.
Leave to cool completely before serving in slices.