Wash and hull the berries if needed. Place them in a saucepan with 1 cup of water.
Bring the berries and water to a boil over medium heat. Reduce the heat and simmer for about 10 minutes, until the berries are soft and have released their juices.
Stir in the granulated sugar and cook for another 5 minutes, stirring occasionally, until the sugar is dissolved.
In a small bowl, mix the cornstarch with 2 tablespoons of cold water to form a slurry.
Gradually stir the cornstarch slurry into the berry mixture. Cook for an additional 2-3 minutes, stirring constantly, until the mixture has thickened to a pudding-like consistency.
Remove the saucepan from the heat and let the Rødgrød cool to room temperature. The dessert will continue to thicken as it cools.
Enjoy your meal!