This recipe is from the Serious Eats website. I've tried this and loved it. It is referenced in the recipe for Slow Roasted Beef Tenderloin. Note: This calls for "prepared horseradish". He's referring to chopped horseradish in vinegar. He also has a recipe for this.
Ingredients
1/2 cup heavy cream
1/2 cup crème fraîche or sour cream
1/2 cup prepared horseradish (see note above)
2 tablespoons chives
1 tablespoon freshly squeezed lemon juice
Kosher salt, to taste
Freshly ground black pepper, to taste
Instructions
1. In a medium bowl, whisk heavy cream until thickened, but not yet at soft peaks.
2. Fold in crème fraîche, horseradish, chives, lemon juice, salt, and pepper. Refrigerate for 30 to 60 minutes before using, or transfer to airtight container and refrigerate up to 2 weeks.