Icelandic Happy Marriage Cake

Instructions

1. Prepare the rhubarb filling
   In a medium saucepan, combine the chopped rhubarb, granulated sugar, cornstarch, and lemon juice. Cook over medium heat, stirring frequently, until the mixture thickens and the rhubarb softens, about 5-7 minutes. Remove from heat and set aside to cool slightly.
2. Make the oatmeal crumble
   In a large bowl, mix together the rolled oats, flour, brown sugar, baking powder, salt, cardamom, and cinnamon until well combined. Add the cold butter cubes and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
3. Assemble the cake
   Preheat the oven to 350°F (175°C) and grease a 9x9-inch (23x23 cm) baking dish. Press about two-thirds of the oatmeal mixture firmly into the bottom of the prepared baking dish to form a crust.
4. Add the filling
   Spread the cooled rhubarb filling evenly over the crust layer, making sure it reaches all the edges.
5. Top with remaining crumble
   Sprinkle the remaining one-third of the oatmeal mixture over the rhubarb filling, creating a crumbly topping. Press down lightly to ensure it adheres to the filling.
6. Bake the cake
   Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and the filling is bubbling around the edges.
7. Cool and serve
   Remove from the oven and let the cake cool in the pan for at least 30 minutes before cutting into squares. Serve warm or at room temperature, optionally with a dollop of whipped cream or vanilla ice cream.