In a small bowl, stir together the sour cream, mayonnaise, horseradish, mustard, garlic, chives and pepper. Taste and add a pinch of salt if desired. For a thinner consistency, whisk in a bit of heavy cream (you can also use milk).
You can serve the sauce immediately, but it will taste even better after sitting for 15 minutes, once the flavors have had a chance to mingle a bit.
Serve with steak, potatoes, or on a sandwich! You can optionally top with chopped flat-leaf parsley for presentation. Leftovers will keep in an airtight container in the refrigerator for up to 5 days (possibly longer, just keep an eye on how the chives look).
If you don’t like mayonnaise, you can omit it and use 5 tablespoons sour cream. I’m not a huge fan of most mayo brands, but Blue Plate is very good. There’s no funky aftertaste, just a nice tanginess.You can finely mince the garlic with a knife, but I recommend grating it on a microplane zester if you have one. It helps release the juices from the garlic so they infuse the sauce more.