This is a traditional French condiment and it’s so easy to make.
Whisk heavy cream and sour cream together in a bowl.
Cover loosely with plastic wrap and let stand in the kitchen or other reasonably warm spot overnight, or until thickened. In cold weather this may take as long as 24 hours.
Cover and refrigerate for at least 4 hours, after which time the crème fraîche will be quite thick.
Sweeten with icing sugar if using as a dessert condiment. Unsweetened, it can be used to thicken soups or to make savoury dips.
You just need to allow time for the mixture to thicken (8 hours or overnight).