Stuffed Apples with Crumble Topping

eating apples (preferably a sharp and rich variety like Jazz or braeburn)

lemon juice

blueberries

caster sugar

constarch

unsalted butter (cold, cut into small cubes)

plain/all purpose flour

caster sugar

double/heavy cream (you can replace with whole milk if you want to lighten it up a bit)

egg yolks

cornstarch

caster sugar

couple of drops vanilla extract