Rub flour, ground almonds, and butter together in a bowl until the mixture resembles breadcrumbs, then stir in sugar.
Work the egg into the mixture with your fingers to form a soft dough.
Shape the dough into a disc, wrap in cling film, and chill in the fridge for 30 minutes.
Preheat the oven to 200°C.
Roll out the pastry on a lightly floured surface.
Cut out 12 discs using an 11cm fluted cutter and line a muffin tray.
Heat the milk in a pan and beat egg yolks with sugar in a bowl until creamy.
Pour the warm milk onto the egg mixture and stir well to create small bubbles.
Transfer the custard to a jug and fill each pastry case.
Sprinkle ground nutmeg into the centre of each tart.
Bake for 25 minutes, reducing the heat to 180°C if necessary for the final 10 minutes.
Cool the tarts in the tin for 30 minutes before removing from the moulds.