While the two share light similarities, such as the cut of beef used (flank steak), they are quite different. Beijing beef is lightly coated in egg and cornstarch for a crispier texture, while Mongolian beef is tender when cooked. Mongolian beef is typically milder in flavor, but in some recipes you'll find dried chili peppers which add to the heat level. It's also made with a savory hoisin-based sauce, whereas Beijing beef sauce is a combination of sweet, spicy, and tangy flavors.