prepared (that is, pickled and grated) horseradish root
a sprinkle of cayenne (optional)
Mix in proportions to taste. Stir till creamy. Sorry, but that's what we do. No measuring: (“Is this enough mustard?” “Nah, let's make it a little darker. Put it another teaspoon. What's it look like?” “More cream?” “Uh, I guess so. I dunno. Hey, who wants to test this? Hallo? Hallo???”) Top with a sprinkle of cayenne for color.
Some prefer the straight grated horseradish root on the roast beef; others a creamy sauce. To each his own.
How ever did this root get its name? Surely horses don't eat the root. Its greens, maybe? Nope, the horse of horseradish means ‘large, coarse’ as in horselaugh and horsebean.