equal parts sour cream and heavy cream
sour cream (A lower fat content, and more likely to curdle if boiled with an acidic ingredient.)
yogurt (Will definitely curdle when boiled.)
To make your own: Warm one cup heavy cream to about 100°, then add one or two tablespoons of sour cream, cultured buttermilk, or plain yogurt (make sure you buy a brand that contains active cultures). Allow the mixture to sit at room temperature for at least nine hours before refrigerating.
can be substituted for Créme Fraîche.