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Even though Arby's has diversified its menu over the years with toasted subs and deli-style sandwiches on sliced whole wheat bread, it's the thinly-sliced roast beef piled high on hamburger buns that originally made this chain famous. Since roast beef and horseradish go so beautifully together, Arby's created this delicious mayo-based horseradish sauce as a spread for the roast beef sandwiches. It also happens to be great on your homemade sandwiches too, but it just isn't cool to hoard handfuls of those blister packs to take home with you. So, with the help of this secret formula, you can clone as much Horsey sauce as you want. First step: get out the blender. You'll need it to puree the horseradish into the mix so that the sauce is smooth and creamy like the real deal.
Source: Top Secret Recipes Unlocked by Todd Wilbur.