Preheat the oven to 200ºc/180ºfan and line a 9x9" square tin with parchment paper.
In a large bowl, add the rolled oats, plain flour, light brown soft sugar, baking powder and unsalted butter.
Rub the ingredients together with your fingers until the mixture resembles bread crumbs
In a pan, add prepped rhubarb, with the sugar and lemon zest and juice and vanilla
Heat on a low heat until the rhubarb is starting to soften slightly
Add the cornflour and stir through, heating for 1-2 more minutes
Leave to cool for ten minutes
Pour 500g of the oaty crumble mix into the lined tin and press down firmly.
Pour the filling onto the oats, and spread.
Sprinkle over the rest of the crumble mix
Bake in the oven for 30+ minutes, or until golden and beautiful.
Leave the bars to cool fully before portioning
