Add mayo, dijon mustard, and vinegar to a small bowl.
Cut off a 2 inch piece of horseradish root and peel it.
Using a microplane, grate the horseradish until you have a 1/4 cup. Add to the bowl.
Take the whole garlic clove and grate that into the bowl.
Mix everything until well blended.
Place in the fridge for at least two hours before using to allow the flavors to meld (overnight is best).
Store in an airtight jar and it will keep for up to two weeks.