The key to re-creating the Big Mac is obviously perfecting the sauce. Some say it’s simply Thousand Island, but the clever folks at McDonald’s deserve much more credit for this masterpiece we crave. In order to re-create the sauce, I had to ensure that I had a supply that I could taste on its own—away from the other great flavors this burger stacks. So I requested “extra on the side.” The friendly woman behind the counter didn’t flinch. She quickly produced a sundae cup half-filled with the neon, salmon-colored sauce. When I took a big sniff, all I could smell was a chemicalesque aroma. Tasting, on the other hand, yielded that wonderful flavor. Several tastings and I was convinced. The secret ingredient? Mustard. Simple yellow mustard. Add that and a pinch of sugar to a Thousand Island–style sauce, and you’ll be surprised how closely it resembles the real deal.