Preheat the oven to 180°C (160°C fan).
To make the crumble topping, put all the dry ingredients into a bowl and mix together. Add the cold crushed butter and rub in with your fingertips to a breadcrumb consistency.
Spread onto a tray and bake in the oven for 25-30 minutes, checking and stiring every 10 minutes until golden. Leave to cool slightly, then break up into a crumble texture.
Meanwhile, toss the apples, rhubarb and sugar together with the vanilla paste and lemon zest and juice. Cook in a saucepan over a medium heat for about 15-20 minutes until the apples are tender and broken down. Transfer to an ovenproof dish.
Top with the cooked crumble and flash in the oven for a final 15 minutes before serving with custard or icecream.