Preheat the oven to 190°C, fan 170°C, gas 5. Whiz the flour, icing sugar and butter in a food processor until the mixture resembles breadcrumbs. Transfer to a large bowl, add the egg yolk and water mixture and stir with a palette knife. Bring the mixture together with your hands to form a dough, adding an extra teaspoon of chilled water if dry. Roll into a ball, wrap in clingfilm and leave to rest in the fridge for 30 minutes.
On a lightly floured surface, roll out the rested pastry to the thickness of a £1 coin. Carefully line a 24cm fluted, loose-bottomed tart tin with the pastry, leaving about 1-2cm hanging over the edge. Prick the base with a fork and place in the freezer for 10 minutes to firm up.
Line the pastry with greaseproof paper and baking beans or uncooked rice and bake 'blind' in the oven on a baking tray for 15 minutes. Remove the baking beans or rice and bake for 5-10 minutes, until golden and crisp. When the pastry has cooled a little, use a small sharp knife to trim the edges. Brush out any excess crumbs with a pastry brush and set aside. Reduce the oven to 180°C, fan 160°C, gas 4.
Meanwhile, in a large bowl, whisk together the remaining yolks, the whole egg, sugar, cream and vanilla seeds, then carefully pour into the cooked pastry case. Put on a baking tray and bake for 30-35 minutes, until just set with a slight wobble in the centre. Leave to cool for about 10 minutes in the tin, then sprinkle with the nutmeg. Great served warm or cold, with pouring cream.