Heat the cream on the stovetop until slightly warmer than room temperature. (about 100 degrees with a thermometer).
Remove from heat and whisk in the sour cream.
Pour into a glass jar. Place the lid on top of the jar but don't tighten it. Put the jar in a warm room for at least 4 but up to 12 hours. It will be thickened and you can use it then (especially if using as a sauce in a dish such as enchiladas), or refrigerate for another 4 to 12 hours to thicken even more.