Similar to sour cream but not quite as thick, French crème fraîche is a heavy cream slightly soured with a bacterial culture. (It's much better than it sounds.)

If you don't have crème fraîche, you could use sour cream. But if you have the time, why not make your own? Mix 1 tablespoon of buttermilk with 1 cup of heavy cream and let it sit for a day in a warm spot (about 80 degrees). The bacteria in the buttermilk will keep the cream from going bad. After 24 hours or you've got your crème fraîche. Put it in the fridge, where it will keep for a week.