For the Marinade:
Mix all of the ingredients except the steak in a large bowl. Add the steak, toss and turn to coat. Refrigerate for at least 30 minutes and up to four hours.
For the Lighter Mongolian Beef:
Heat oil in a large skillet over high heat. When hot, add beef and the white parts of the green onion, cooking quickly to sear beef and stirring often. Do not overcook; remove from the pan when the beef is still slightly pink in the center.
While beef is cooking, add cornstarch to the bottom of a large cup. Add a little bit of soy sauce, stirring into a paste, then add more, stirring to avoid clumps. When smooth, stir in the water, Sherry or Mirin, and hoisin.
Add the mixture to the hot pan, stirring with a whisk, and simmer until desired thickness is reached. Add beef and any juices back to the pan along with the green parts of the green onions and simmer briefly.