In a medium saucepan, combine fresh rhubarb, caster sugar, and vanilla extract, stirring gently. Cover and cook over medium heat for about 15 minutes until soft.

Preheat your oven to 200°C (400°F). Pulse the rolled oats in a food processor until slightly finer. In a large bowl, combine pulsed oats, plain flour, brown sugar, ground cardamom, and baking soda.

Cut softened butter into the dry mix using a pastry cutter until it resembles clumpy crumbs. Crack in an egg and mix until it looks like wet crumble.

Press two-thirds of the dough mixture into the base and sides of a lined 20 cm (8 inch) cake tin. Spread the rhubarb compote over the crust, then crumble the remaining dough on top.

Bake in the preheated oven for approximately 25 minutes or until golden brown.

Remove from the oven and let cool for a few minutes. Slice into squares and serve with whipped cream or vanilla ice cream.