Preheat the oven to 200ºc/180ºc fan
Add the oats, flour, sugar and butter to a bowl and rub together to a breadcrumb texture
Add the frozen berries to a new bowl and mix with the sugar, flour and lemon zest
Pour this into a small/medium baking dish
Sprinkle over the oat crumble
Bake for 40+ minutes
Add the egg yolks, sugar, cornflour and vanilla to a bowl
Whisk together to light in colour
Slowly pour in hot milk, whisking well
Add back to the milk pan, and stir on the heat until thickened
Serve with the freshly baked crumble
