A traditional crème fraîche with a kick from horseradish, Chef Michael Symon serves this atop his recipe for beef cheek pierogies.
Heat the heavy cream in a small saucepan over low heat to 105 degrees. Remove from the heat and whisk in the buttermilk. Cover the pan with plastic wrap and leave in a warm place (90-100 degrees is optimal; the cooler the temperature the longer fermentation may take) for 36-48 hours.
When the cream has thickened, chill it in the refrigerator for a few hours, until cold, or for up to a week. Before serving, fold in the horseradish, chives, pepper, and salt to taste.