In a dish, mix the mayonnaise, sour cream, horseradish, Dijon mustard, garlic powder, and salt and pepper until well combined and not lumpy
Adjust seasonings based on your preferred taste. The sauce can be served immediately, although many people prefer serving it cold. If serving cold, store in an airtight container but only up to a maximum of two days in the refrigerator before serving in a shallow saucer
Notes
If you want to add a little more kick, consider adding a splash of vinegar, a pinch of chili flakes, or a few drops of Tabasco sauce.
If you’re one of those people who find the horseradish flavor overwhelming, consider adding some brown sugar, maple syrup, or honey.
If you're feeling a bit more adventurous, consider using herbs, spices, or freshly sliced vegetables to give your sauce a bit more punch.
If you’re grilling your prime rib steaks and want to incorporate the smoky flavor to your sauce, add paprika to the sauce.
Remember that horseradish has a really strong flavor, so prepare a little bit at a time. Also, remember not to buy too much at the store else you end up with too much horseradish you can’t use.