In a medium saucepan, combine chopped fresh rhubarb, superfine sugar, and vanilla extract. Cook over medium heat for about 15 minutes, until the rhubarb softens.

Preheat your oven to 200°C (400°F) and prepare your baking tin by greasing it with butter.

Place the rolled oats into a food processor and pulse until they reach a slightly finer texture.

In a large mixing bowl, combine the blended oats with all-purpose flour, both sugars, ground cardamom, and baking soda. Mix thoroughly.

Add the softened unsalted butter to the dry ingredients and work it in until the mixture resembles a crumbly texture.

Incorporate a room-temperature egg and mix until just combined.

Press about two-thirds of the dough into the base and up the sides of your prepared cake tin.

Spoon the prepared rhubarb compote evenly over the crust.

Sprinkle the remaining crumble mixture over the rhubarb compote, covering it while leaving some gaps.

Transfer the cake to the preheated oven and bake for approximately 25 minutes, until the top is golden brown.

Let it cool in the tin for about 15 minutes before transferring to a serving plate.

Slice into generous pieces and serve warm or at room temperature with whipped cream or ice cream.