Heat the oven to 180C/160C fan. Use the pastry to line a 20cm round tart tin, a loose-bottomed one is easiest. Press it into the flutes, trim the excess and chill the case for 10 minutes in the fridge.
Warm the milk in a small pan until steaming, but do not let it boil.
Whisk the eggs, sugar and vanilla in a jug, then pour in the warm milk, whisking as you go.
Put the pastry case on the oven shelf, then carefully pour the custard in, right up to the top. Grate a good snowy covering of nutmeg over the whole surface.
Bake for 30-35 minutes until the custard is just set but still has a slight wobble in the very centre when you nudge the tin.
Cool completely in the tin before removing and slicing. The custard firms up as it cools and shrinks back slightly, which makes cutting clean slices much easier.