Scrape the seeds out of the vanilla pod and mash them together with a little of the sugar.
Cut the rhubarb into slices about 1 cm thick and stir them together with the sugar.
Mix all the dry ingredients for the crumble and add a good pinch of salt.
Add the butter and rub the mixture between your fingers until it begins to clump and the butter is worked into the dry ingredients.It should look like a coarse, lumpy mixture.
Tip the rhubarb into a pie dish or similar and spread the crumble over the top.
Bake the rhubarb crumble in a preheated conventional oven (top and bottom heat) at 200°C for about 25 minutes.
Leave it to cool for 10 minutes before serving with cold crème fraîche or vanilla ice cream.It can also be served cold.