Thoroughly clean the berries and rhubarb. Cut the rhubarb into smaller pieces.
In a saucepan, combine the cleaned berries and rhubarb with water, sugar, and vanilla sugar.
Bring the mixture to a boil over medium heat and let it simmer for 3 minutes.
Reduce the heat to low, allowing the mixture to simmer for an additional 10 minutes. Stir occasionally.
While the mixture is simmering, mix cornstarch and 2 tbsp of cold water in a small bowl to create a thickening agent.
Gradually add the thickening agent to the simmering mixture in the saucepan. Stir well after each addition to ensure even distribution.
Increase the heat to medium and bring the mixture back to a boil for another 3 minutes until it thickens.
Remove the saucepan from the heat and let the red berry porridge cool slightly.
Pour the finished red berry porridge into a bowl or individual serving glasses and refrigerate for at least 4 hours or until it cools and sets.
Serve the chilled red berry porridge with a generous amount of cold heavy cream on top.
Enjoy your delicious rødgrød with heavy cream!