Preheat the oven to 350 ℉ (180 ℃) with convection (fan).
Cut the rhubarb and the apples into bite-sized pieces.
Place the rhubarb and apples in a baking dish, add the white sugar and vanilla sugar and mix well to cover the fruits thoroughly.
Bake the fruits in the oven for 10 minutes.
In the meantime, mix the flour and brown sugar in a large bowl, and rub in the butter to form crumbs. To check if the consistency is right, press the crumbs into a ball in your hand: if you can make a ball, and it crumbles with a bit of pressure, you are good to go. If you cannot form a ball, add more butter. Or, if it doesn't crumble back, add more flour.
When are 10 minutes have passed, take out the baking dish and spread the crumbs on top. Put the dish back in the oven and bake for 30 to 35 minutes. Done!
