Ingredients
1/2 cup mayonnaise
2 tbsp sour cream
2–3 tbsp prepared horseradish (grated fresh, mixed with vinegar)
1/2 tsp sugar
1/4 tsp salt
Pinch of white pepper
If you're starting from fresh root, you'll need to make "prepared horseradish" first:
Peel and grate the root (do this outside or near a fan — fresh horseradish fumes are strong).
Immediately mix with 2–3 tbsp white vinegar and a pinch of salt. The vinegar stops the heat from building further and preserves it — add it within a couple minutes of grating for the sharpest bite, later if you want it milder.
Let sit 10 minutes before using.
Then stir 2–3 tbsp of that prepared horseradish into the mayo/sour cream base above, adjusting to taste since fresh horseradish is often much hotter than the jarred stuff. Let the sauce sit in the fridge an hour before serving so the flavors meld.