Regardless of its name, Mongolian beef is in an American Chinese dish. You will commonly see it across popular Chinese restaurants across America, but most likely will not find it on a menu in China.
Mix the sliced flank steak with soy sauce, sesame oil, rice wine, eggs, corn starch, and salt and pepper until combined. Marinate in the refrigerator for 30 minutes.
In a medium-size bowl whisk together soy sauce, brown sugar, oyster sauce, rice wine, water, rice wine vinegar, and corn starch until mixed and smooth. Set it aside.