In a medium-sized heavy-bottomed pot, heat the sour cream over medium heat until warm, stirring occasionally.
Gradually add the flour to the sour cream, stirring constantly, until the mixture thickens and separates into a smooth mass and some fat begins to separate. This should take about 5-7 minutes.
Slowly pour in the milk, about 1 cup at a time, stirring well after each addition to ensure the mixture stays smooth.
Continue cooking and stirring the porridge over medium heat for 10-15 minutes until it thickens to a creamy consistency.
Add the salt and sugar, stirring to incorporate evenly. Taste and adjust seasoning if needed.
Remove the pot from heat and let the porridge cool for a few minutes before serving.
Serve the Rømmegrøt warm in bowls, topped with a dollop of butter (optional), a sprinkle of ground cinnamon, and granulated sugar to taste.
Notes
For a richer flavor, use high-quality sour cream. Adjust the sweetness and cinnamon to your preference.