Heat oven to 200C/180C fan/gas 6. Roll the pastry out to about 1cm thick, then grate over a dusting of nutmeg using a fine grater. Fold the dough in half, then roll out again, to the thickness of 2 x £1 coins and large enough to line a 20cm loose-bottomed sandwich tin with some overhang. Press the pastry into the tin (leaving the excess draped over the sides), put onto a baking sheet and chill for 10 mins. Line the pastry with baking parchment and fill with baking beans. Bake for 20 mins, then remove beans and paper. Bake for another 15 mins until golden and sandy all over. Trim the edges of the tart with a sharp serrated knife.
Turn oven down to 150C/130C fan/ gas 2. Whisk together the eggs and sugar in a large bowl. Put the cream, milk, vanilla pod and seeds into a saucepan and bring to the boil. Pour onto the eggs, whisking as you go. Sieve into a jug. Put the baking sheet onto the pulled-out oven rack, then pour in the custard, right to the top. You may not need every last drop. Grate over a nice layer of nutmeg, then slide gently back into the oven and bake for 1 hr. When it’s ready, the tart should be set and pale golden on the top, and have just the merest tremor in the centre when you jiggle the tray. Cool completely, then serve in slices.