Put the plain flour and butter in a bowl and rub together with your fingertips until they resembles breadcrumbs. Mix in the icing sugar and a pinch of salt, followed by the egg yolk. If the pastry feels too dry to form a dough, add 1-2 tbsp water.
Roll the dough out between two sheets of baking parchment until it forms an approximately 28cm circle. Put in the fridge for 20 mins to firm up, then rest over a buttered 23cm tart tin. Push down into the corners and trim the top so there’s a 1-2cm overhang of pastry. Don’t worry if it breaks slightly, just patch up any tears with the offcuts. Prick the base several times with a fork, then chill in the fridge for 45 mins. Heat the oven to 200C/180C fan/gas 6.
Lay a sheet of baking parchment over the chilled tart case and fill to the top with baking beans. Blind bake for 15 mins. Remove the baking beans and parchment and bake for another 10-15 mins until golden brown. Leave to cool in the tin, then trim off the excess pastry around the top with a small knife.
Meanwhile, bring the milk and vanilla to the boil in a saucepan over a medium heat. Beat together the egg yolks, caster sugar and custard powder until well combined.
Pour the hot milk over the egg and sugar mixture, whisking continuously, until the mixture is smooth and well combined. Return the mixture to the pan over a medium-low heat. Stir continuously for 2-3 mins until the mixture is very thick. Leave to cool slightly, giving a whisk occasionally so it doesn’t go lumpy.
Pour the custard mixture into the cooked pastry case, spreading to level it out. Put it in the fridge for 2 hrs to set completely.
Pour half the lemon juice into a medium bowl of water, then finely slice the apples into the bowl, using a mandolin if you like. Sprinkle with the caster sugar and microwave for 3-4 mins on high, then transfer into a bowl of cold water with the remaining lemon juice.
Wrap 3-4 slices of apple around each other to form a rose shape – rolling up the first one, then wrapping the other slices around it. Press the apple flowers into the custard, working your way around until the whole tart is covered.