Make the Shortcrust Pastry
Chill your butter until it is cold enough to grate. Rub the grated butter into the flour, using only your fingertips. You should end up with a mixture that looks like breadcrumbs. Mix in the sugar, salt and nutmeg, then stir in the egg yolks and white. Work the mixture with your hands until it comes together in a cohesive ball. Wrap the dough in plastic film and put in the fridge to chill for 30 minutes to an hour.
Blind Bake the Pastry Shells
You can use this recipe to make 4 individual tarts or one big one. It's best to use tins with removable bottoms. Whichever one you are doing, you'll need to butter the inside of your tins generously. Roll out the pastry to a thickness of 3-4 mm - just under a quarter of an inch. Lay the pastry into the tin and carefully press it down into the bottom and edge of the tin. Be very gentle with it - this dough is quite fragile and can easily tear. Trim off the excess dough but leave about a centimetre (half an inch) overhanging the edge - this will stop the pastry from shrinking when you bake it. Prick the base all over with a fork. Put the pastry cases in the fridge and chill for half an hour.
Preheat your oven to 160°C (320°F) if it's a fan oven, 180°C (356°F) if it isn't. Remove the pastry cases from the fridge. Cut squares of greaseproof paper big enough to line the cases. Scrunch up the paper, then unscrunch it - this makes it easier to shape into the cases. Fill the cases with baking beans or rice, place in the oven and bake for 20 minutes. Then remove the beans or rice and the paper lining, and bake the cases for a further 10 minutes. Now paint the inside of each case with beaten egg and bake for 2 minutes more. This effectively forms a seal over the pastry and stops the filling from soaking in.
Set the blind-baked cases aside to cool a bit. Turn the oven down to 120°C (248°F) for fan, or 140°C (284°F) if not fan.
Make the Filling and Bake the Tarts
In a small saucepan on medium heat, mix together the cream and the milk, and bring to a gentle simmer. In a large bowl or jug, whisk together the eggs, yolks and sugar. Now whisk in half of the simmering cream and milk. When this is done, whisk in the rest of the cream and milk.
Place your tins on a baking sheet and put it in the oven. Pour enough custard mixture into each one to come as close to the brim as you can get it. Generously sprinkle with nutmeg and bake for 35-45 minutes. The cooked filling should still have a slight wobble when you take it out.
Finish the Custard Tarts
Let the baked tarts cool down completely, then use a serrated knife to cut off the excess pastry. Remove the tarts from their tins and scoff the lot.