Peel and roughly chop horseradish, then add to a food processor with peeled garlic. Pulse until finely ground. Leave the mixture uncovered for about 5 minutes, outdoors if possible. During this time, an enzymatic reaction develops the horseradish's characteristic heat. The longer it rests before adding the vinegar, the hotter it will become.
Transfer the horseradish mixture to a bowl. Add vinegar, sour cream, mayonnaise, blue cheese, salt, and pepper; whisk until blended.
Notes
Be careful when processing fresh horseradish. The fumes can be surprisingly potent.
For a smoother texture, use a high-speed blender or bottled prepared horseradish.
For a milder sauce, skip the resting period and add vinegar while blending.
Use fresh horseradish root for the best flavor; however, prepared horseradish can be substituted.
Vinegar controls the heat. Adding vinegar stops the enzymatic reaction that creates horseradish's intense heat.
Adjust the consistency to your liking. Add a little more sour cream for a thinner, tangier sauce, or more mayo for a richer, creamier texture.
Let the sauce rest before serving. Refrigerating the sauce for at least 30 minutes gives the flavors time to meld.
Store in an airtight container in the fridge for up to a week.