Tempering Sour Cream (Hartowanie)

Polish soups like żurek, ogórkowa and barszcz biały are finished with sour cream, and it curdles if it hits boiling soup cold. The fix is hartowanie, tempering. Put the sour cream in a bowl with a spoonful of flour if you want the soup thicker. Add a ladle of hot soup and whisk until smooth. Add another ladle and whisk again. Then pour it all back into the pot, off the boil, and stir. Heat it gently without letting it boil hard. Full-fat 18 percent sour cream holds up best.